It's confusing, to say the least. Seems like UA "continues" its flights a lot. Some UA domestic flights have 8xx or 9xx flight numbers, which are usually used for transpacific and transatlantic flights, respectively.

I remember flying UA 891 LAX-NRT and connecting to UA 853 NRT-TPE, both in C. Despite the different flight numbers, I flew on the same aircraft, which happened to be my namesake N117UA. It's even more confusing how UA keeps on renumbering the Asia-Asia2 flight numbers while keeping the USA-Asia flight numbers unchanged. For example, the flight number for the aforementioned NRT-TPE flight has changed from 853 to 891 and 801.