I know what you're talking about. In sewing it usually happens when the dart is too long; a proper dart should end no closer than 1" from the bust point even in something highly tailored. So that may be a possibility. Depending on the fabric, you might be able to have a tailor redo the dart... but there's the risk that the old stitching and press lines will still show.
It's also possible that it's just too tight. A dart is a point, by its nature, and if it's under too much strain it can be accentuated. The type and color of fabric affect this as well.
I have this issue sometimes. Partly it's because I have a bust point that's naturally a little lower than the fit model's. There's enough room for me, but I'm just a little lower. (Probably because I have a small band and larger cup size.) I know my bras fit fine, so I just have to decide how obvious it is and how much it bothers me. This is often one of those things that are really obvious to us in the mirror, but invisible to virtually everyone else. If you're having this issue in a lot of items, do make sure that your bras are well fitted and not worn out. It may be worth it to go bra shopping and just try on new things, with one of the offending shirts. It would be an interesting experiment, if nothing else!
