My guess is this is partly due to the slide mounts and partly due to the light table's diffuser surface reflecting the image back to the scanner. Images not dependent on critical sharpness will scan fine for web work, after downsizing and resampling, or will make acceptable proof prints. But don't expect it to substitute for a dedicated film scanner. Finally, there's a definite difference between the performance of the 1200 ED and Mustek's similar USB flatbed, with otherwise identical specs. But it doesn't handle scans with my homemade transparency adaptor any better in this mode, and not even as well in reflective mode.
Scans are grainier and contrastier, with more apparent scan lines.
The 1200 ED is a terrific flatbed for the money. Naturally, Mustek has discontinued it. Scanning a photograph can be slow, but the results are pretty good for an inexpensive scanner. I had an HP5100C that broke down way before its time. DOA from internet auction house.