Hmm, try comparing the condition the brushbar, clutch, filters and air path around the DC03 to the DC07 and 04 and you might not be so surprised at all.
If not, then its got to be because of the fact the last vacuum that cleans the rug up will pick up left over dirt, its like comparing a DC07 to an Argos value range upright, and the Argos value upright will pick up just a little bit more.
This is because of the dirt that gets trodden down by daily walking over and the wear and tear of the carpet, so it needs a thorough vacuuming, I tried it with a DC04 silver lime model, I vacuumed an area rug, emptied it like 5 or 6 times and it still picked up the dust in the carpet