Either Mouse Lock or GUIs Not Centering Properly?

Hey guys,

I've noticed that when I center a GUI on the screen - say for example, a GUI object with position (0.5, -50, 0.5, -50) and size (0, 100, 0, 100) - the GUI object is slightly below the center of the screen. At least, the point on the screen that the mouse locks to when in first person or in MouseLock. I initially thought this was a bug with Roblox's topbar, but I was wondering if there is anything else causing this that I do not currently know about.

Thanks