I'm not currently in Edinburgh at the moment so I don't have access to Black's Surnames of Scotland which is really the definitive source for issues relating to Scottish naming practices but I can explain this generally in relation to the customary practices of Scottish clans.

Clan Ross is not unusual in not subscribing to a 'Mac' form of [someone's] name, first of all. Other notable examples of this are Clan Campbell, Clan Fraser of Lovat, Clan Grant, Clan Colquhoun, Clan Donnachaidh, Clan Urquhart, and Clan Stewart (among others). The most notable 'Mac' clans (historically speaking) are the Clans (Mac)donald (often, MacDonald and Donald are used interchangeably), Mackenzie, Macleod, and Mackay, and although other smaller 'Mac' clans existed, many of them were incorporated as subsidiaries of larger and more powerful groups. Thus, the MacEwens were the hereditary bards of the chiefs of Clan Campbell and they did not have a 'chief' of their own. They were a sept of this larger clan and were absorbed by it much as you'll find MacAndrews absorbed by the Clan Ross.

Gaelic naming practices traditionally operate patrilineally. That is, one is the 'son' or 'daughter' of [the father's name]. Clans like Clan MacDonald derive their name from the third chief of the clan, Dòmhnall Mac Raghnuill (who, himself, was identified as 'Donald, son of Ranald'). Generally, when clans adopt a 'Mac' form derived from an earlier chief, they are either doing it to distinguish descent (i.e., when multiple clans share a single ancestor they may choose to differentiate themselves by making explicit which of that ancestor's sons established their particular clan) or to draw an obvious connection to an ancestor or former chief who accomplished something great in his lifetime. But that would have been at the chief's discretion and he was able to do this because, as chief, he would almost always have been a direct male descendant of these earlier ancestors. Things get a bit fuzzier when you have non-related people assuming the clan surname as a demonstration of loyalty or affiliation. It's why so many Scottish clan associations today are raising funds to undertake DNA projects: they want to see just how many Campbells, etc., are actually genetically related to the current (or last living) chief and how many likely just assumed the surname for the reasons outlined above.

But, back to Clan Ross, specifically. If you look at a map of Scotland, you'll see the present-day county of 'Ross and Cromartyshire'. Back in the Middle Ages, this was divided up between Ross and Cromarty and to the west of that general area lay Wester Ross from whence the first chief of Clan Ross first came. Fearchar mac an tSagairt (Fearchar, son of the priest - his family were hereditary abbots the church in Applecross) rose to prominence in the thirteenth century when he quashed a rebellion in his territories against Alexander II and was subsequently knighted by the grateful king and made the first earl of Ross. And this gave rise to the clan name. There never was an individual in the clan whose forename was 'Ross' that could have been turned into a patrilineal Gaelic surname using the 'Mac' form. Rather, the first chiefs held the earldom of Ross for four generations before it passed to a daughter who married a Leslie and thus took the earldom away from the 'Ross' surname. From there it passed by marriage to the MacDonald Lords of the Isles and then, when that fell, it was absorbed back into the crown. But, the original Rosses continued to define themselves as 'Ross' to demonstrate that the title had originally belonged to them. Yet, since they were no longer the actual earls, they had to differentiate themselves from the earl of Ross and they did this by appending a territorial designation at the end of the name. Thus, you get chiefs whose names were things like David Ross of Balnagowan.

I should note, though, that surnames were an ambiguous concept in Scotland right through the seventeenth century. Because of the Gaelic influence, some Scots continued to have fluid surnames that illustrated ancestry rather than anything fixed in the way that became common in England and elsewhere fairly early on. It helps to think about the Gaelic meaning of the word 'clann' as 'children of' though. Clan Ross are the 'children of the [first] earl of Ross'. Clan Donald are the 'children of Donald'. Clan Campbell are the 'children of Cambeul' (the Campbell surname originated as a Gaelic by-name or description of the first chief, Colin the Great, who evidently had a 'crooked mouth'. Whether this means his face was physically crooked in some way or just that he was as oily as many of his descendants is up for debate though!)

One last bit of information before I close this relates to the Gaelic 'titles' assumed by many chiefs of clans whose names don't employ the 'Mac' form. The chief, himself (or herself, today), will often assume a personal, hereditary 'Mac' form that illustrates the unbroken line of descent from the first chief of the clan. So, the chief of Clan Fraser of Lovat is known, in Gaelic, as MacShimidh (son of Simon). The Chief of Clan Campbell is known as MacCailean Mór (the great son of Colin). However, since I don't have Black's Surnames with me here, I can't check to see if the chief of Ross assumed a similar style. If he has, though, it would likely be something like MacFhearchair (son of Fearchar). Incidentally, one of the reasons Clan Ross is not MacFhearchair generally is because there is an entirely different clan, anglicized as Farquharson, who already use that form.

Anyway, I hope this helps somewhat. Recommended reading on the topic, if you can get your hands on them are George F. Black's Surnames of Scotland and the two histories of Clan Ross that were written by Alexander Ross and John Ross in 1932 and 1972 respectively.