Most Helpful Girl
Just like with regular clothes, different brands fit differently.
You might buy a medium shirt at target and it fits perfectly, but when you shop at Hollister you might need a large (for example). Same with bras. A 34C at Wal-Mart is most likely not going to be the same as a 34C at Victoria's Secret.
Most Helpful Guy
Maybe different bras have different sizing to it... or she wants to push it upwards
Who knows? ask her