I've been having trouble buying bras recently - when I buy one that I think is my right size (38b) it's way too tight around my ribcage, with some I can't even close the clasps at the back, they don't meet.
Just went online to try ordering some from Amazon, and glanced at the Amazon size chart and I'm now 
38b, according to their size chart, measures 33.5 inches underbust, 36 inches overbust.
I always thought the 38 inches referred to the measurement around your ribcage, under the breasts, and the cup size was according to how many inches above that measurement 'around the bust' was.
No wonder I can't get a bra to fit, apart from old worn-out ones :(