There was a good write-up on here a while back, I believe by sbm12, which demonstrated how UA.com doesn't always show the 'cheapest' options by default, but instead has to be forced to.

I believe what you're seeing on Google flights is indicative of this. I've replicated the same thing via Hipmunk many times. By forcing connections or fare class, I'm often able to find cheaper options for the same city pairs.

In some cases, I've even had forced "W" fares for the same itinerary be cheaper than the lowest available default fare.