There are 3 IVs that range from 0 to 15 and they are (once the bugs are fixed) rolled for independently.
For Pokemon caught in the wild (i.e. ignoring starters, nest and egg hatches) the IVs are generated as simple random numbers, so any particular pokemon has a 1 in 16 chance of having any particular stamina, attack or defense IV.
However, because there are three IVs, the chances of getting above a particular combined perfection threshold combines multiplicativly: i.e. there are 16x16x16, or 4096 different possiblecombinations of IVs.
As a perfect (100% score) pokemon requires 3 separate rolls of 1 in 16: a perfect pokemon should occur in 1 in 4096 catches. A 97% pokemon requires two IV rolls of 1 in 16, and 1 roll of 2 in 16 - i.e there are 3 combos (15,15,14), (15,14,15) and (14,15,15), so the chance of getting a 97% or better pokemon is (3+1) in 4096.
A 95% pokemon adds an additional 6 combos (i.e. pokemon who's IVs are (14,14,15)* or (13,15,15)*), so the chance there is 6+3+1 or 10 in 4096.
And yes, this will look like a bell curve when plotted.