The Baffling "Gmail Wiggle" Baffles

I do not understand the design thinking behind engineering in this action. It doesn't help the user discover that there's more to the "More", while the motion distracts the eye and moves other potential targets. Not to mention that it's inconsistent: sometimes the wiggle happens, sometimes it doesn't, and it's not clear why it should happen when the user hovers over one mailbox and not another. (Clarification: it may be difficult to see in this low-res version, but before the wiggle, the "More" text is partially obscured from below, though still readable, while the wiggle reveals the entire word, but nothing below it.)