look at my earlier post about repair issues.
2400 is less than the physical resolution of the scanner according to the specs. Try a scan of both at 6400. or at least a reflective scan. That will give you 1:1 pixel to sensel resolution and see what shows up. At significantly lower resolution, this sort of issue can be averaged out or even skipped over depending on which sensels are used.
So i reckon you will still have a good scanner for documents and such. And, yes, after 10 years it is not surprising.