Is there some font difference between OSes, perhaps?

There are always tremendous differences between operating systems, you know - Wolfram Inc. has to compile the front end as well as the Mathematica kernel against the actual operating system:

• Windows (all flavours) is rather a message broker than an operating system - despite they sucked in all the Digital Equipment operating system people after the decline of Digital Equipment; but this was the deciding move to make Windows NT a worldwide success, e.g. see DaveCutler
• Linux and Mac OS are rather ordinary operating systems - kernel focussed - but had to fight to keep up with Microsoft Windows windows ...