Unfortunately I have recently received some upvotes, which means that new bugz are uncovered...
TL;DR: if there's a day, on which you receive upvotes on post, then you check reputation tab and after some time receive upvotes on the same post again, rep tab tells you wrong amount of new rep gain (ONLY in the "time" mode)
Reputation tab shows everything correctly if there aren't any new reputation changes since the last time you have visited the rep tab. Otherwise, there could be problems.
I'll give you a couple of examples based on my reputation history for March, 30 (that's yesterday, yeah).
Let's visit my reputation tab, select the "time" mode and check March, 30. Here's what you will see:
There're 5 upvotes and 1 downvote for the post about topbar.
Now let's assume that the last time you have visited the tab, it was March 30, 20:00 UTC on the clock. You can click this link, since I know how to use a time machine and hence it's repro'able.
Oh wow, how cool is that? So how many upvotes have I got in total?
Conclusion: if there're some reputation changes that you haven't seen yet, the time tab shows them wrong. I prefer using the time tab sometimes, since it highlights the unseen rep changes with yellow which allows you to check how many votes ago I checked it.