I have read that the Romans used old reverse dies with new obverse dies. This looks like it could be one.
The reverse looks double-struck to me. The lack of legend on the reverse may be because of the strong strike on the obverse, which sometimes resulted in not enough metal on the reverse filling in the die. I don't think that the reverse die looks especially worn...
As stated above, the reverse is a double strike. The reverse dies have a tendency to wear out faster then the obverse dies which is why you will often see really sharp obverse with a worn reverse. But that's not the case here, the die shifted between strikes.