It may seem funny at first glance, but airlines use forecast models based on the mountains of data they accumulate from past years of experience. The number of seats they will release into U and the time at which they will do so varies by route, season, month, week, day of the week, time of the flight, and numerous other factors. So comparing two flights--even ones that look similar to you--is really comparing apples to oranges when it comes to projected F loads.

That said, on a flight with 0 F seats sold (though remember, the seatmap is not a fully accurate picture of how many seats have been purchased, as you can buy a ticket but not assign a seat), there is a decidedly-greater-than-0 chance that U will be released at some point, so make sure to follow dave1013's instructions and sign up for a notification service.