In the Preview monitor my caption is comfortably within the TV area I have defined (5% margins).
But when viewed full screen (or in the exported video) it is not.

I've so far found no setting that changes this puzzling difference. But changing font size from 18 to 17 did make both look the same. Changing to 19 did so too, but in that case they were both touching the edges.

It seems to be a quirk, probably occurring only with rare combinations of text length, font type and font size. So I'll be sure never to rely on my PAL (half) preview for precision.

Or for better quality: https://dl.dropboxusercontent.com/u/4019461/MEP2014Text-Inconsistent.jpg