With only 4 digits for flight numbers and all their flights and codeshare partners, UA is very short on flight numbers. You'll also see "nonsense" direct flights like PHX-EWR-SEA on the same day not because they think anyone would fly that for PHX-SEA, but to conserve flight numbers.

There's also a theory that the day-to-day scrambling of flight numbers is to hide their poor on time performance.