Same route, same day, same airline -- different flight times?

This is probably a newbie question, but it's been bothering me -- UAL operates several daily nonstop routes from RAP to DEN. On the day that I'm looking to fly, the 3pm flight is booked at 79 minutes, while the 8am flight is booked at 91 minutes. That seems like a pretty sizable difference for such a short flight.

So, same airline, same route, same day, short haul domestic, but one is more than 15% shorter than the preceding flight. Are they factoring in morning traffic at DEN? Or is there something else going on?