Sister sizes?

Someone please explain bra sister sizes to me. I just find it so confusing lol. I've always wore 34B VS bras. I go in a few days ago and she tells me she thinks I may be a D and tells me to try on a 32D. Of course I did and it fit just fine. Some 34B's fit better and some 32D's fit better. But she got all kinds of excited for me when the D fit. Isn't it basically the same thing? Lol