That's very odd.
I've been trying out the whole mana gain and such. With this I started with a level 1 char with no attributes what soever, and then raised it. I started at 70, with an increase of 15 per level.
Maybe it has to do with the difficulty setting? I've only ever played at hardcore, so I wouldn't know if difficulty settings make any difference...
Either that or you just have a buggy game?
Also, I played mages that played purely on int, from the beginning of the game (with at the end of act 1 sth like 7 strength, 9 agility, and 31 intelligence) and I don't think I saw a dramatic decrease in mana gain by the end of the act. (you think I would have noticed...)
