How does boil time directly affect ABV% & IBU? Dogfish Head makes IPAs called "60 Minute", "90 Minute", and "120 Minute". According to Wikipedia:


  Their names refer to the length of the boil time of the wort in which the hops are continuously added.


There is also a limited edition "75 Minute" which is really a blend of the 60 &amp; 90 varieties.

What I find odd however, is that these numbers also roughly correspond to the beers' ABV% &amp; IBU.

Again, from Wikipedia:


60 Minute: 6.0% ABV, 60 IBU
90 Minute: 9.0% ABV, 90 IBU
120 Minute: 18% ABV, 120 IBU
75 Minute: 7.5% ABV, (no IBU listed)


Generally, the ABV% is about one-tenth the boil time in minutes, while the IBU is directly equivalent. Is this naturally due to the chemical reactions which occur during this part of the brewing process, or must Dogfish Head be doing something in particular which leads to an abnormally tight correlation here?
First, it's important to note that the boil time is not the only thing that increases with each of those beers.  Dogfish Head continually hops during the boil, and the boil extracts the alpha acids from the hops, giving the beer bitterness, so a longer boil with more hops results in a wort that is more bitter. 

Here is where the additional alcohol comes in:  To counteract the extra bitterness, the beers that have boiled longer have both additional malt added to balance the bitterness, and are given additional fermentation and conditioning time to allow the yeast to convert all of those malt sugars into alcohol.  In 120 Minute IPA's case, it spends an entire month in fermentation, resulting in an exceptionally high level of alcohol at the end.