Umm a Dyson Light Ball gets a 'C' rating for carpet. Shouldn't you be asking why a Dyson doesn't get an 'A' for the claims they make about their 'wonderful technology'?
Also, the X7 has a different programming sequence and adjust itself with different logic. It does not work in the same way as their previous models did with their heads going to and down continuously. You also forgot to mention the X4 Boost models get a 'B' rating on carpet. The same will be on the X7 Boost models obviously.