I've also noticed this, and originally I figured it was just to provide a bit more visual variety to the relatively drab mine levels. It has since occurred to me that a signature element of the walkway levels is the hanging lights, which also appear in Stop & Go Station and Loopy Lights.
Honestly I'm not sure if it was an artistic decision or a technical one that led to these artificially-lit mine levels having the walkway lights in the background, but either way, I like it.
Edit: I've adjusted our
Stop & Go Station map to reflect this observation; it was originally auto-assigned the standard mine background by a script I created to help put the maps together.