Oh god I have the opposite issue. My boobs will NEVER do that. I have Pectus Excavatum which creates a valley in the middle of my ribcage, moreso than normal cleavage (ever wonder how a girl with such small boobies got rockin cleavage!! Deformities yay!)
Basically this means my boobs always always always create a gap in the bra, looks like it doesn't fit me. But it doesn't matter what size I try on, the bra will always have a little gap. It's so annoying. Even worse when I think I've bought a bra that doesn't do it and then 1 day into wearing, out comes the gap.