I think what you're seeing here are roughly 1px differences between where the browser decides to lay things out. Combined with the fact that the eye icon is placed a little low, but that would likely be down to how it's exactly defined in the font. If you look at other button icons, you'll see the align better in most cases (covering from above capitals to the descenders).
I have made a very small tweak to the CSS here, though it may not really look much different. Such I'm not sure what to even mark this bug. Can't fix?