I suspect that it is just another facet of yield management. AC most probably want to realize as much revenue for each flight as possible and need to balence the sale of tickets from the various buckets with a desire to not alienate their frequent flyers.

All conjecture is probably pointless. Somewhere they have locked a way an old man with a long grey beard, dressed in long white robes and sandals, their yield and load management guru. Occasionally he checks the omens and portents and decides how many to put into each seat bucket.

My last trip LHR-YVR and back again were showing availability in most fare classes up to the morning of departure. However, when it came to it Y was full and pax were opuped to J. Y and J were full LHR->YVR and had plenty of space YVR->LHR. In both cases there was plenty of I inventory.

Chris.