Originally Posted by
Jaid314
naturally. the roll of 3d20 will tend towards the middle values much more than the rolls of 1d20...
that is, there is an even distribution from 1-20 on a d20, with a 5% chance of each number occuring (in theory). the odds of rolling a 10 or 11 are actually equal to the odds of rolling 1 or 20.
on 3d20, assuming you had that exactly equal distribution of numbers, you would only get a 3 in 1 out of every 8,000 rolls or something crazy like that (that is, you'd need to hit 3 consecutive 1/20 chances, so 1/20/20/20 = 1/8,000).
in comparison, to get a 4, it's something like... you could roll: 1,1,2 = 4, or you could roll 1, 2, 1 = 4, or you could roll 1, 1, 2 = 4, so three times as likely if i'm not mistaken... or 3/8,000 which is still not a very high chance.. for a 5, it could be 1, 1, 3 or 1, 3, 1 or 3, 1, 1 or 2, 2, 1 or 1, 2, 2 or 2, 1, 2, so 5 chances out of 8,000 (much higher, but still not high). this sort of thing continues on, with theoretically most rolls coming out right near the middle of the "bell curve" so to speak. as a result, you see a lot more variation in numbers close to the theoretical average value of 3d20 + 60 (ie 91.5).
this is why a 5 point difference from 70-75 (close to 3%) is worth so much less than the 5 point difference from 90-95 (close to 15%)
and of course, starting from 75 and onwards (the lowest value where the "first" roll can be 13 and up), each AC point tends to be slightly less valuable than it otherwise would be, although this is mostly overridden by the overall trend for AC to matter close to ~91.