Originally Posted by CO_Hoya

Here is Sheldon Brown's bottom bracket cribsheet (link).

That is almost certainly a French-threaded bottom bracket, and therefore the "fixed" cup has a tendency to loosen up over time if not secured very well.

It is not unusual that this cup is very well-fixed indeed, if it has not loosened up on you. It may be wise to service the rest of the bottom bracket but leave that cup in place.