MechaPilot
Explorer
If the second set of arms are meant to have similar range of motion to the first, pectoral muscles or muscles of a different name performing the same function are certain to be present... as to the additional mammary glands that appear to be present, I would rather not speculate upon reasons for their inclusion beyond this: drawing something that you can't actually even get a real-world reference for is not easy.
Well, they should have no shortage of reference for pectoral muscles without mammary glands to look at when drawing the lower set of arms. However, it's not like it's a big deal or anything. It's just something that I thought was a strange choice by the artist.