Originally Posted by
Lux Flyer
I mean it's probably more test the waters at the lower fare as opposed to give away a ton of seats. Having watched quite a few flights it's pretty common as they feel the need to open lower buckets they will sit at [X]9[Y]0 for a day or two then go to [Y]1 before eventually expanding to [Y]3, and [Y]7 a few days later if no one bites when it is [Y]1. Additionally with their usage of continuous pricing it makes this less relevant if they put something at [X]9[Y]1 as long as you're booking through a direct UA channel as you'll get things like described a few posts previous where W4 for 5 passengers booked into W still.
Acknowledging that we don't precisely know what drives the specific inventory calls, what I've seen is UA serially open X1, either in the original fare class or the next higher. I just had this happen last week - flight was Q1 when I checked, by the time I was ready to buy a bit later, it was H1. Along the same lines (but different facts and different manifestation), I've bought one ticket at P1 and it briefly goes to P0 before UA opens one more P fare (P1 again). In both cases, for a 2 pax PNR, UA is forcing one fare class higher.