Not sure what causes this behavior, but at Turn 71, the top power (a God-like AI) in the game only has 1 mining starbase, and is severely lacking mining resources.
It has 10 free administrators to use, and plenty un-mined resources in its zone of control.
Turn 1 and Turn 71 save files can be found here:
I hope Brad can shed some light on this.
EDIT: corrected typos in the file link