It’s a combination of two things:

(a) Revenue Management is intentionally a black box. Airlines work hard to make sure that their fares are as unpredictable as possible. We will never know all of the inputs that they use to figure out where to set inventory levels.
(b) It’s the weekend. UA does most of its schedule management on the weekends, and that can mess with inventory as they make changes.

Note that the correlation you’re looking for, between the number of tickets they’ve sold and the price, doesn’t exist. (See: point (a) above). It’s entirely possible for a flight to be empty and for RM to think that discount inventory should be zeroed out, because they think that they can sell those seats for more money if they wait.