It remains a sloppy fit, but as alluded to here that might be of benefit.
It's to prevent the actual hole locations from preventing proper alignment.
Technically, at least one rail should be mounted against a machined shoulder, and you don't want the bolts pulling the rail away from that shoulder.

And the fact that many others do it that way, doesn't make it right. Just look at the thousands of poorly built machine on the internet, with thousands more people eagerly copying their designs.