I don't remember seeing these buttons before... I'm pretty sure the "Users" page used to look like the other tabs with the options being in a row with red underlines to highlight which tab is active on the page.
Now there are buttons but for some reason, the "reputation", "new users", and "moderators" all hover highlight in dark blue, making the text difficult to read while "voters" and "editors" don't.
I'm guessing this is a fragment of the responsive design, as it does resemble the user pages on Stack Overflow. It's worth noting that this seems to have been rolled out to every site that I've checked, not only Meta Stack Exchange but the odd hover color only appears here (as far as I can tell).
So, I did some more poking around this morning and I realized that the reason that "reputation", "new users" and "moderators" have a different hover highlighting is because they're visited. Not quite sure how I missed this but it might help in determining how to fix it.
This is global across all sites with the buttons. Here's the page on M&TV:
As you can see, I've tested this by clicking on the "editors" and "moderators" tabs, which have dark circles around them after being "visited" but the other three do not because I haven't clicked on them. "Editors" is darker grey because it's the tab that's currently active.