Very interesting question! I think there a couple misconceptions we can adress before we move to the bigger question of why they were so skilled. First, it should be noted that Norman lords were not terribly more advanced in terms of fortification style than the Anglo-Saxons that they conquered in England. England at this time possessed a highly complex system of fortifications that spanned the entire Kingdoms, often referred to as Burhs. Begun by Alfred the Great, the Burhs were towns and cities that were intentionally fortified against raiders and invaders, and were manned, built and repaired by the population of the surrounding countryside. This system was partly responsible for the eventual expansion of the Kingdom of Wessex to encompass much of modern day England. We don't have a ton of information of what these fortifications looked like, but they certainly would have made use of the most common building material (wood) but could also have built upon existing Roman fortifications.

When the Normans arrived, the Burh system was still in place. The Norman innovation was to heavily fortify the countryside as well as the population centers. Small Motte-and-Bailey castles proliferated the English countryside in the post-conquest years, as new Norman lords attempted to solidify their stronghold on a restive English population. I'm talking literally hundreds of these things. Part of the reason they were so common was because they were pretty easy to set up - a motte-and-bailey consisted of a large mound upon which you would build a tower surrounded by a palisade (wooden wall) which would also encircle a small area that would house the men, horses and buildings necessary to keep the small fort running. These were not the great stone structures of Caernarvon, Kenilworth or Dover. They were simple, easy to construct but allowed the Normans to keep watch over the conquered English. The large stone structures that you mention only really started coming about in the 12th and 13th centuries, as stone-working and military structure evolved to a point that allowed Kings and Lords to construct larger and more complex castles. But it's important to understand that comparable stone fortifications were being built at the same time across Europe. And Caernarvon (as well as the other castles Edward I built in North Wales) were actually built by a Savoyard, James of St. George.

In retrospect we can see that Norman castles were not truly revolutionary - they improved upon an already existing system of fortifications (Burhs) which they did not observably surpass in design or construction. There were more of them, sure, but they were not radically superior to your run-of-the-mill Burh. By the time that they started building larger and more complex castles primarily of stone, this was really the practice in much of Western Europe. For instance, expansive stone fortifications existed in France (Carcassonne is a possible example) before and during the time that the Normans were constructing their most impressive castles.

Did that answer your question at all? For further reading you might enjoy:

Creighton, Oliver (2012), Early European Castles: Aristocracy and Authority, AD 800–1200, Debates in Archaeology, London: Bristol Classical Press

and

Higham, Robert; Barker, Philip (1992), Timber Castles