51studebaker: Dimensionally, and as RIS mentions, the center of the platen is more accurate than the outer edges.
This intriguing observation inspired me measure DPI variation across the platen on my brand new Epson V600 scanner. So I wasted a couple hours this evening taking several dozen 2400 DPI scans of a glass microrule placed at different locations.
Horizontal DPI varies just as you suggested. In the center 3 inches of the platen a 50mm scale measures 4744 pixels (about 2410 DPI) at every vertical location. When placed in the outer 3 inches, a 50mm scale measures 4759 pixels (about 2418 DPI) at every vertical location.
Vertical DPI on my scanner is almost precisely 2400 DPI regardless of location on the platen.
My wild guess is that vertical DPI accuracy is a function of scanner head movement accuracy while horizontal accuracy is mostly a function of optical accuracy of the scanner head itself.
The new scanner is a significant improvement over my 10 year old Epson V300! It will be interesting to redo this experiment in a few years and see if the results change.
For most practical measurements a delta of 20 DPI from 2400 DPI is irrelevant; however, it could be significant for things like like differentiating rotary vs flat printings, small vs large perforation holes, or making Kiusalas gauge measurements.
Regardless, the little OCD devil on my shoulder prompted me to make a few software changes to my image processing workflow to compensate for the delta anyhow... ;)