Problems with default column width and width to pixel ratio

This problem consists of two parts.

1. I get the wrong default column width. From Microsoft documentation, it should be 8.43 character units. I've also seen 10.71 on some devices in Germany, which might be the standard here. However, I get a default column width of 10.38 on my PC and 10.07 on my Surface. I understand that the default column width depends on the default font and font size, but my settings are Calibri 11, just as on any other device, so that cannot be the issue here.

2. Wrong width-to-pixel ratio. I understand that the width of columns in pixels is calculated based on the column width in character units and the default font and font size. When using Calibri 11, I should get a ratio of 0.133875, corresponding to a width of 10.71 with 80 pixels. However, my numbers are all over the place:
My PC: 10.38, 88 pixels -> incorrect ratio of 0.118...
My Surface: 10.07, 160 pixels -> incorrect ratio of 0.0629...

These problems might or might not be related, any help would be much appreciated.