Join Date: May 2005
Location: various cities in the USofA: NYC, BWI, IAH, ORD, CVG, NYC
Programs: Former UA 1K, National Exec. Elite
Posts: 5,485
Open inventory does not imply that any fares exist for that bucket. Even if fares do exist, the rules might preclude purchase (e.g. min. stay).
Edit: For example, take a look at flights departing in, say, 2 hours. Almost all buckets may be open, but you almost certainly cannot buy a T fare.
FortFun beat me to it by < 60 seconds....