How come user A ends up with more rep when he receives negative feedback than when he receives neutral feedback (essentially no feedback)
A single user can't both up-, down- and not vote all at once, on a single post. You can upvote and unupvote/downvote shortly after, but that cancels your upvote. So, starting with user A and B:
If user A receives negative feedback from user B, their reputation would be 3, and they would end up with less reputation than they'd have with no feedback at all.
If user A receives no feedback (or an upvote/unupvote, downvote/undownvote) from user B, their reputation would remain 5.
If user A receives positive feedback from user B, their reputation goes up by 10 points to 15.
After having received positive feedback, A is at 15 reputation. Then user C comes in, with negative feedback, and A ends up with 13 reputation. Which is less than the 15 they started with before the negative feedback. So the negative feedback did cost them reputation.
The error in your calculation is that you assume
an upvote + a downvote = negative feedback, while you should think of it as only
downvote = 'negative feedback'.