I'm sure you're aware that Tokyo is a hub for United, futhering that concept, hubs often have "hub times" - or times when a large number of flights converge on that hub at a particular time, then depart from that hub at a particular time. In this case, UA837 arrival from San Fran happens to represent roughly the begining of the Tokyo daily hub time, and it's departure to Bangkok happens to represent the end of that daily hub time. By roughly 6:30 (depending on Season) basically all United flights will have left Tokyo for the evening.
Most United flights tend to arrive between 2-3:30, and most United flights tend to depart between 5-6:30PM. Keep in mind that arriving into NRT
can often encounter quite lengthly air traffic delays, not to mention the fact that simply flying 5,000 miles across the ocean can also result in landing times differeing from thier scheduled times.
Based on my numberous times through NRT
on United, this length of times is just about right. Gives one enough time to enjoy the more important things in life....like the Tokyo Red Carpet Club's Automatic Beer Machine.