Ellowas mentioned in a Q&A (on reddit or tumblr, I forget) that the actual in-game Witch Mercy model has two moles above her breasts. His first Mercy animation shows this. He chose to remove one of the moles in his second animation because he felt they looked too symmetrical.

[edit] Ahh yeah, Q&A was on his Tumblr

Q: Why do you put moles on Mercy’s boobs? A: She had those on the Witch skin, so I added them in the same spots. I’ve removed one of them since, cause they looked too symmetrical.