Cup sizes are relevant to band sizes. So what is often measured as a 36b is often a slim 32 F.
You get 80% of your support from the band, do off you measure 32, then you need a 32. Adding 4" makes no sense. Would you do it with your trousers?
Women know that the old way of measuring made no sense, and are starting to find new ways of measuring that make sense (now shops need to catch on). The reality is that 36a/b and C is a rarity and 32FF is a common size.

Then you have no idea how a bra should be fitted.