I agree with what you're saying but the High Street is just as guilty of this. I can be either a 12 or 14 in the same shop - just depends on what type of garment it is.
It makes trying on less enjoyable as, for example, if I can take 6 garments in, I'd probably take the item in a 12 AND 14, rather than 6 completely different items.
However, my size 10 friend bought a size 16 jumper in Jane Norman the other day and it fitted her perfectly! As they only go up to a size 16, I wouldn't be able to buy anything in there.
No wonder young girls have eating disorders.