Microsoft Word sizes everything for the printer. What you see on screen is merely an approximation. This actually makes sense, since printers will be more consistent than screens.
But I'm designing when I'm in Word, it's not necessary or convenient to do all of my work at 1:1 scaling, that's what the ruler is for.
Compounding the problem is that the operating system relies on the monitor reporting its physical size correctly via EDID and for a lot of vendors (most notoriously Dell) this isn't guaranteed either.