Did United tell you that you were not changing planes? There are some intineraries that have the same flight number for two different segments but they change planes at the connecting airport.
If you do change planes in SFO be aware that United is now temporarily (for 18 months) operating out of two different domestic terminals while they redo their gates, terminal 3 (main united terminal) and terminal 1 (only select flights, mainly to/from some southern Cal cities and LAS, I think), and there is a bus bridge set up airside (post security) to transport passengers from one United terminal to the other. If you have to use the bus bridge you'll need to make sure you leave enough time to get from terminal to terminal. Oh, and they don't tell you that your flight is in another terminal, the only way you know is by the gate number on your boarding pass. You go to the gate and get on a bus to the other terminal. So if you do change planes don't mull around until the last minute because you think your gate is nearby based on the terminal map, you might get there and find you have a wait for a bus to go to the other terminal. The gate numbers for the bus bridges are fixed so you can tell by the gate numbers if you have those.
Edit:
Actually, I think they print on your boarding pass now if the bus gates apply to you.