Vertical alignment of text in sheet cells is inconsistent, specifically, "Align Middle" does not actually put the text in the vertical middle of the cell. 

Consider these cells aligned bottom, taking note of the amount of padding is between the text and the bottom of each cell:

Now look at when the same cells are vertically aligned to the top:

Notice how the top padding is far less?  So, what happens when we try to vertically align to middle?  See:

Notice how each cell's text is closer to the top?  I believe this is because of some margin/padding issue somewhere because while none of it is correct, it is consistent:

I tried this at a higher zoom (not browser zoom, but the actual Sheets zoom tool) to see if it is a zoom issue, but it's not.  Zoomed in we see the same behavior.

Bottom aligned:

Zoom Top aligned:

Zoomed middle aligned:

Taking the zoomed Arial cell as an example, and doing some comparisons of the vertical spacing we see this (the numbers are approximates found by drawing rectangles to visually fill the space. They numbers don't add up exactly for that reason):

The two major things of note:

My conclusion here is that your cells have, for some reason, approximately 8px more padding on the bottom than on the top - a padding that exists "around" whatever space the text moves in when its vertical alignment is altered, leading me to think you have something like this for the cells: