It sounds like you have a lot of programs marked as KUID (Keep Until I Delete), either in the To Do List, the Now Playing List, or both...
If so, then what you're seeing is typical, because TiVo calculates used disk space based not only on existing programs, but by what programs that can be deleted from the NPL (which it cannot delete KUID programs to make room for new programs, so TiVo considers that space not available for future recordings), as well as KUID programs yet to be recorded in the To Do List.
TiVo will still record the shows in your To Do List as long as there is available room and/or programs it can delete from the NPL (that are not marked as KUID) at that time.
If so, then what you're seeing is typical, because TiVo calculates used disk space based not only on existing programs, but by what programs that can be deleted from the NPL (which it cannot delete KUID programs to make room for new programs, so TiVo considers that space not available for future recordings), as well as KUID programs yet to be recorded in the To Do List.
TiVo will still record the shows in your To Do List as long as there is available room and/or programs it can delete from the NPL (that are not marked as KUID) at that time.