I don't think I have a particularly narrow rib cage, but I require a 28 band bra. I can't be that rare, but it's very hard to find bras this size without spending £££.
M&S and the like don't carry many styles under a 30 band. I'm sure a lot of women actually require a 28 but settle for 30 or maybe even larger because they have to.
It's easier to find things in very large sizes than it is in smaller, even though 28 isn't particularly tiny anyway.