Found my size but why one fit me and the other doesn't - 28FF US

Hi! I recently posted to get help after the calculator gave me 28DDD/G size and I ordered two bras from Amazon but neither of them fit. Someone commented that I should try 28F/28FF sizes. Sadly 28F cup didn't fit me either (side tissue spill) but 1 out of 2 of the 28FF ones did. It felt beautiful on me!

But my question is about this difference. Wouldn't you expect to be able to order that size freely once you think you've found it? They were both definitely 28FF but the band on the wrong one was so tight and badly fitting. Both of them are from Freya so it's the same brand too!