possibly because the adjustment is being made to 10, 12, or 14 bit data before saving out as 8 bit.
THe same adjustment in PS is being made on 8 bit information.
Very few scanners/software allow software analogue adjustment of the scan itself.
This is what SF says on the subject. As I read it the software is controlling the scanner to deliver a full 16 bit histogram with adjustments in place. I have tried for some time to get a clearer statement about this from them but my "good" contacts at SF are long gone. I think though that regardless, Tylers point is pretty true, that as long as you are working in 16 bit with low noise scans, it doesn't matter much.