Likely due to the packaged font not being used on the mac machines (Calibri) and the Macs own fall back version of that font, which it insists on using, has a slightly different spacing for a few of the characters.
Two options exist:
(1) I find a way to embed the font into the document in a way that the Mac likes
(2) we just switch the document font to something which is used across all browsers (websafe) that looks the same as Calibri. (e.g Carlito)