Well, basically: chains that carry a limited range of sizes have an investment in sizing you into the wrong bra -- if they size you out of their range, then you likely won't be buying any bras from them.

Specialty stores are more invested in getting women into the actually right size for them, even if they don't carry the size, because they're generally run by people who care. Although that's no guarantee, either.

Basically: boobs are complicated and if I were rich I'd just wear bodices and/or corsets all the time. Because fuck modern fashion, the back support is fantastic.