Interesting observation. I wonder if it's related to the method the puzzle creator used to create the keys:
There is no pattern. It is just consecutive keys from a deterministic wallet (masked with leading 000...0001 to set difficulty)
IMHO I think it is. Even if the creator of the puzzle didn't do it purposly the software he used most probably was adjusted to generate the most complicated keys within the each given range.
Sorry, but I think your math is actually wrong. You are using the range 1..2^bit to calculate the occurrence in the range, but you should be using the range 1..(2^bit - 2^(bit-1)). That would explain why all your results are occurring at > 50% of the range.
Ok. For example #60
60 | 0xFC07A1825367BBE | 1Kn5h2qpgw9mWE5jKpk8PP4qvvJ1QVy8su | 1152921504606846975 | 98.5% | 1135041350219496382
1135041350219496382 is how many percents out of full range 1152921504606846975 under your calcs?
I was also wrong on part of my response. The range I stated (1..(2^bit - 2^(bit-1))) is the right cardinality (number of values), but the values given by that range are wrong. The range should be 2^(bit-1) .. 2^bit.
Not completely sure about the math, but I think the percentile for bit 60 using this range is
(1135041350219496382 - 2^59) / 2^60 = .48 = 48%