It is probably due to the fact United figured out that more leisure passengers will do their search for air tickets during off-work hours, thus UA will intentionally increase the price for the same ticket when searched during off-work hours as well as over the weekend? For example, for the same ticket in a future date, you might have a better chance for lower price if you do the search and purchase the ticket on Tuesday or Wednesday (the travel date does not necessarily need to be Tuesday or Wednesday), rather than do the same on other days of the week, especially over the weekend?