Seven Fifty7 is right, UAL doesn't have to have LAX-SFO exclusively Shuttle. They could throw in a mainline 757 or 320 in there during peak times and still charge Shuttle prices. Some of the Shuttle flights on the route don't even carry the designated Shuttle 2000 series flight number and often originate as 777/757/747/320 flights in other cities and then transfer the flight number to the 737 for the LAX to SFO or SFO to LAX hop.
Just six or seven years ago UAL used to routinely fly 727, DC-10 and 757s between LAX, OAK, SFO and SJC. This was before SW of course.