I believe what you will find is that there are a few pmUA 777s without crew bunks. They are not often used on the 12+ hour flights that require four crew members, but when they do, two seats (2A and 2K) must be blocked. Except for take-off and landing when all four are in the cockpit, they rotate two up front, and two on rest.

I was on one such flight PEK-IAD a few months ago. It was a last-minute substitution, and the purser told me they had to downgrade someone who had been previously been upgraded to give them the 2nd crew rest seat.