I think it's not that uncommon. I've read about it, forget what I've read. Some can be, if I remember, a result of surgical disfiguration. Another a2k friend told me about a kind of wasting after radiation. I have higher and lower boobs from three b/c surgical techniques.... one is smaller and I'm glad enough to have it. Nice, I can remember a surgeon friend joking about it a few decades ago, re mal-functioning boob enhancements.
Anyway, it also might be normal. Or it may be a symptom of something. Time for google.