Stat inflation definitely occurs as the game proceeds, but that seems a little extreme to my eyes, at least for the first 100 years. My current game (admittedly only 50 years in) has only one character whose base traits average in excess of 10 among single women aged 0-25. It seems like a lot of the characters near the top of your list have that 594 number--is that a dynasty identifier, or the province of Sarkel? There might be a freak strain there.
I do think stat inflation becomes very noticeable toward the end of the game for systemic reasons. The A.I. is very conscious of stats when it searches for wives, and the vast majority of characters never marry, so an extreme form of sexual selection takes place. In addition to the visible stats, the health and fertility of the characters also tends to improve over time, making it much harder to marry into property late in the game, because sick/infertile lines naturally produce fewer children than the healthy/fertile ones.
I'm not sure what the solution is. I don't know how inheritance works precisedly--do you start out with base stats that are the average of your parents adjusted upward or downward randomly? If so, maybe it could be modified so that the start point for all stats would be averaged with 5, to prevent extreme drift. Another possibility might be to make the frequency of negative inbreeding effects more common, to reduce the chance of superstrains emerging in particular dynasties.
edited to add:
Some of them could be bastards or maybe offspring of bastards. In my last game, there was a little sheikhdom in Transoxania which was full of superhumans...one ruler after another with all stats 12+, and several office holders with primary traits of 20+. I thought it might have been because the muslim states had been almost totally wiped out, so that the few that remained had a huge selection of courtiers and little competition.
