RAM right or wrong?

I've always understood that hard disc sizes get exaggerated by sellers
dividing by 1000 instead of 1024. I also understand about usable disc
space being less than quoted sizes due to sectorisation.

But I've never come across this before..

My new Dell laptop was sold as having 1024MB RAM. Both Vista and the
BIOS report the amount of RAM as 894MB. Dell CS have used both the
reasons in the first paragraph as reasons for the discrepancy. The other
suggested reason is that some of the RAM is shared with the video card -
but why as the Radeon Xpress 1150 has 256MB of its own?

Can someone explain please?