For a lot of reasons!
* 1) They likely have their own brand (and respectively their own sizing grid)
* 2) Every bra is different. Every one.
* 3) A lot of big chain stores don't have sizing that goes above a DD so you have to compensate with the band to make everything... fit
The average cup size range we see is F-H and I'm not joking even a little bit; The problem with this is that brands like La Senza and Victoria's Secret (no hate though) don't always go this big so you're forced into a bigger band.
This can obviously cause a lot of dominoes to fall; Your band riding up, straps slipping off, boobs popping out... they're all symptoms of the wrong size of bra.
Easily (usually) remedied by finding the right size and shape of bra for your body.