The reason they do that is that there are different results in terms of which dice rolled which number which get bundled together. If you're not bundling any numbers together - and 2d10 as a d100 doesn't do that - you don't see the effect. Using a 2 d10s as an example there are 100 distinct results. 2d10 ranges from 2-20, for a total of 19 different results, with those near 11 much more common than others. The only way to get a 2 is to roll a 11 (an 11 in a d100), whereas to get an 11 you can roll 1,10; 2,9; 3,8; 4,7; 5,6; 6,5; 7,4; 8,3; 9,2; 10,1 . That's 10 different results, but it's also ten different numbers when using the dice to simulate a d100, so you don't get that weighting.