Twallan had something on his blog today that I think explains this:
Okay, the Skill Gain issue added with University. Follow me on a little journey into adventure, shall we ?
EA adds the Protest system, where performing protests influences certain aspects of the game
One of these protests applies an "Increased Understanding" moodlet to sims in town, increasing their skill gain as long as the moodlet is in place
However to ensure this skill gain persisted when you saved and reloaded, one of the developers needed to change a field in the Skill Manager system
In doing so, they made a little error : They actually needed to change two fields, not just one.
The field they changed involved increased skill gain when your sim has a high mood. So now the game saves that skill gain to the save file as well.
The problem is that every time you save and reload while your sim is in a good mood, it will compound the "high mood" skill gain value.
As one might expect, this issue has a rather obvious effect if you happen to save and reload a lot.
It also mentions a fix that requires one of his mods.