Seems to be a precompute failure. It is pricing the trip to SIN with a stopover in MLE until you click through and choose a return that isn't MLE-SIN-SFO. This will show the correct price ($2,xxx in M).

This is mildly common for very complicated routings where the computer is probably hitting a computation timeout and giving up prematurely. Note that you may also see price differences on similar routings as UA and LH inventory for the same FRA-SFO flights are different, and Google can force ua.com to sell LH native inventory (sometimes).