Do consumer flatbed scanners merely discard data ?

Let's say a scanner has only one sensor, and only one lens. Like my Epson 4990.

When you scan a negative - and specify a resolution that is less than the maximum - doesn't the scanner just scan at max resolution (the only one it can), and discard the extra data ?

On my Epson 4990, I have seen little difference between a 600 ppi scan upsized to 4800 ppi in Photoshop, and an explicit 4800 ppi scan.

I know that the 4990 is really able to deliver something like 2400 ppi at best.

well at least in one dimension it's not just scanning at max and then discarding data. haven't you noticed how the scanning head moves master for low res scans vs high? it can move faster because it doesn't have to stop and pick every pixel. it's not recording that data and then discarding, it's just skipping over it. as for the other dimension, i have no idea.