Not the best capture, but lately noticing upper chest 'divots' and not finding much info on this- genetic structural or a sign of lacking development? As much as I like them want to be sure I'm functional first. Any clues?

As of today, I am a Hollowback Girl!!! (Stock pics, not me). New pose achieved in my first Power Yoga class today. Strong and happened naturally as I tried for a std HS. My instructor was impressed- said she can't even do it (same injury as me)!!!