UA still has high frequency on said route, and they operate different sized aircraft throughout the day. Plus, Virgin America and Southwest moved in on this route. JetBlue added 2x daily flights between SFO-LGB in the past couple weeks. It's not peak travel season. There are no more $39 leisure fares that easily fit into peoples disposable income stash.
For those reasons, it's a real possiblity more flights will have open seats. And if not an explanation for how bad the airline might be hurting now, could be an indicator for the future.