Chibi, it is vanity sizing. Certain retailers will have generous fits (M&S, Monsoon, Gap) others won't (Zara, Mango, Topshop) I even once heard it being advertised on QVC, when they claimed: 'Well, in some shops you may be a L or an XL, but here at QVC you have a pleasure of ordering a size S or M and enjoying a wonderful fit'
. Or something along those lines.
I think LittleRedDragon's response illustrates this practice quite nicely.