Don't know what the plumbing is, but it would appear that EN left to its own devices has two different sizes, 10 isn't quite 10. If you look at the ENML a font size parameter gets added. Just reset the size on the original text and you will get the same result. On the other hand, it didn't change if I re-selected the 'Segoe UI' font. So I'd guess that the font that's created by default and the font that's specified when you change font size isn't created with exactly the same parameters as the default font is.

With Segoe UI/10 showing for font parameters, I then selected the new text, and re-selected '10' from the font size dropdown, and the selected text, while still looking like the original, spread out just a little, as if the spacing between characters increased a tiny bit, maybe by a pixel.

Then I copied that text, hit a newline and pasted it. OK, fine - looks like the same in MS Word.

One curious thing: I set the default font to Segoe UI 10, and created a new note, then typed some text into the note ('Segoe UI 10', how original).

What is evernote default font