It all depends on the route and how their internal systems are allocating inventory and spreading demand. I've seen fare bucket availability vary to extremes just within several hours... T fares would show up and then 2 hours later it was gone and only H was available, then the next day it was back to L. There really is no logical explanation without knowing the inner workings of their revenue control system and inventory system... thats just how it is. Fares can vary a lot during the day and week.