well that's what threw me off originally and why i wasn't more careful about fitting it.
I think its either one of two things. even though its a little too big there's some flex and i was able to push it through the holes about 25% of the way without too much issue. So I'm guessing people are able to get there belts tight without needed to go past 25%. In the picture you uploaded that looks about as far as i was able to go before it got too tight. I also looked at other files and in one of the remix's of this design they even recommend printing it at 98% scale because of the tight fit.
Or maybe there's some variance to the acrylic plates, maybe your plate holes are at 28mm. i got my printer recently so it could be that newer ones have slightly smaller holes.