Yeah that's been the case for a looooong time. I think your actual credits should be correct though (although I've only ever glanced at it to check rather than done the proper maths)
It seems there's something going on with the credits during game purchases. I'd post screenshots in bugs. Someone can start one topic for everything so it's not scattered all over the place :)
The copying the last entry thing seems accurate, the last thing I had before that entry was a 800 ebs login bonus, and then it says I got 800 credits for donating.
The accountant has done that for a while, it copies one of the last numbers and says that instead. I've always found my credits increase by the correct amount though
Sent Eve a mail. Pity the accountant only goes back a week, I'd like to have seen if this is a new issue or if my previous purchases were missing credits too.