I feel like this answer is "no, there is no reason for that to be the sequence you are seeing: it is an accident, as you can see from this careful analysis which demonstrates exactly why it is that sequence"... it is like saying "I noticed a bunch of random processes fall into a normal curve, is there a mathematical reason for that?" and saying "oh no, that is entirely on accident; here, let me show you the math for how a particular kind of random selection causes that exact distribution you are seeing, along with the equations that characterize if... as you can see, no math is involved here: this was just an accident" :/.