Arial Bold letter spacing "S" on Windows

You are seeing here a difference in spacing between characters that is exactly one pixel in size (bravo on having an exceptional eyesight).

Here is how your second example looks like in Word when extremely resized:

I don't believe that the problem is kerning. I think that the problem is the fact that, finally speaking, a pixel cannot be sub-divided - it's either totally included, or not.

In my opinion, when Windows calculates character sizes, there is a problem when the result of the pixel calculation is not an integral number of pixels. In this case, the font glyph generator must either round up or round down the number of pixels that it allocates.

That is probably the reason that when the scale (pt) of the text is changed, rounding may go up instead of down. Specifically, the space was rounded down to 5 pixels in the upper example and rounded up to 6 pixels in the lower example.