If you have a look at the Sumerian King list (http://etcsl.orinst.ox.ac.uk/section2/tr211.htm) you'll see that the translation has kings before the era of the flood living even longer than the purported ages of Adam, Methusaleh, etc. One explanation is that rather than a solar year, the unit of measurement was a lunar year, or a growing season (3 modern months). However, it's also possible that these were deliberate or non-deliberate myths to signify the closeness of the ancestors to the gods. There's some debate on the topic and no definitive explanation.

Because it fits with what was understood to be a 'good' life. Joseph dies at 110 because that's an ideal life time for Egyptians (but note that Joshua dies at 110 as well). The Hebrew ideal life was 120 (partially based on Genesis 6:3), which is why Moses dies at that age.

The other life spans of Abraham (175), Isaac (180) or Jacob (147) are more problematic and are not finalised. Cassuto argues that the numbers are all multiples of 5, or multiples with 7 added onto it. That's echoed by Gervitz who has something like this:

Abraham: 175 = 7x5^2

Isaac: 180 = 5x6^2

Jacob: 147 = 3x7^2

Joseph: 110 = 1x5^2 + 6^2 + 7^2

Note the increasing scale of the squares. Labuschagne disagrees with this number 7 theory, but claims a divine number of 17 (I'm not sure at what point numerology becomes "the meaning you create" vs "what meaning was intended" and I suspect you start going into nutcase territory (cf Eco's Foucault's Pendulum)). Most of the post-diluvian ages are explained like this or along these lines in terms of increasing scale, or composition of previous ages (it gets complicated). The ones that are important to Hebrew thought can be explained like this, the genealogical lists are less important and throw up other problems.

For the longer ante-diluvian life spans, there are theories from dependence on the Sumerian and Babylonian king lists, to Babylonian astronomy, the sexagisimal system etc., the issue being that no one suggestion covers all the ages, you always end up not explaining some of the ages with your chosen system. So the short answer is we don't know really, but have a couple of educated guesses.

Edit: fixed open parenthesis :>

To add to what others have already correctly pointed out, certain character's years-lived equate numerically to developments taking place in the early Second Temple period (622 B.C.E-70 C.E.).

For instance, concerning the figure of Enoch, Genesis 5:21-24 reads:

When Enoch had lived sixty-five years, he became the father of Methuselah. Enoch walked with God after the birth of Methuselah three hundred years, and had other sons and daughters. Thus all the days of Enoch were three hundred sixty-five years. Enoch walked with God; then he was no more, because God took him.

The number '365' is not a coincidence, or at least not by the late Persian period (ca. 4th c. B.C.E.), early Jews are reading it as relating to the solar cycle. The priesthood, run by those claiming patrilineal descent from Zadok who was the supposed first high-priest, operated according to the lunar cycle. This is why the Jewish calendar to this day appears to shift each year.

A mystical tradition developed around Enoch and the number of years that he lived wherein groups that accepted it appeared to operate from the foundation of a solar calendar (i.e.: in direct opposition to the high-priesthood in Jerusalem). A good example of one of these groups was the sect of Essenes at Qumran. There is a tremendous amount of debate whether or not they followed a solar calendar, but in the Dead Sea Scroll literature that they produced, the books of 1 Enoch and Jubilees were recovered. The authors of these works go to great lengths to show the superiority of time's division according to the solar cycle.

All this to say, some numbers may have represented certain features that we can trace a possible development over time. For Enoch, who eventually came to known as "Little God" in some circles for, his age is a likely numerological representation for the solar year.

Edit:

I should have offered a quotation from Jubilees to explain my point. Jubilees 2:9 explains the significance of God's creation of the sun. It says:

And God appointed the sun to be a great sign on the earth for days and for sabbaths and for months and for feasts and for years and for sabbaths of years and for jubilees and for all seasons of the years.

Only the sun is given such description. The moon, after its creation, is not further explained. Enoch is discussed in Jubilees 4 and 6, which 1 Enoch further expands.

I've often wondered if the early characters in the Bible were were given lifespans long enough render a Genesis that would not conflict with the genealogies and accepted age of Egypt at the time.

Other than the problem with throwing out your entire worldview, what is the problem with seeing them as fairly accurate (with a couple of years) recording of their lifespans.

http://kukis.org/Chronicles/1chron01/100104.declining_longevit.jpg

http://pandasthumb.org/images/Sanford_talk_Loma_Linda_decay_of_lifespan_in_OT.png

The ages seem to follow a fairly nice polynomial decrease following the population bottle neck of Noah's flood.