How can you have more used_pages than total_pages when looking at sys.allocation_units?
I suspect it is related to the fact that this DMV is updated asynchronously behind the scenes.  Per the sys.allocation_units BOL page:


  ...the values returned by sys.allocation_units immediately after dropping or truncating a large object may not reflect the actual disk space available


While this implies only drops and/or truncates won't be tracked in real-time, I just ran the same query against one of my environments where nothing is ever dropped or deleted and am getting similar behavior to your results.  I suspect this view just is updated in the background and whatever it is that has to trigger to spawn the update hasn't happened yet.

Hopefully someone else has a better answer as I'd be interested to know for certain as well.