I noticed something strange when looking at reputation changes for this account: the reputation change for September 17th shows up as zero. If you expand it however you see one upvote:
Now I suspected that he might have downvoted ten answers on this day (reputation change from these downvotes doesn't appear in the list for other people's accounts). Problem is, he only downvoted two posts in total so far.
Things get even weirder if you use a different link to have September 17th rating immediately expanded - all the sudden the reputation change for that day is 10:
What's going on?