Perhaps the right mental model is that the action selector gives you Remaining actions, or a filtered subset thereof (or Completed actions).
- Due Soon gives you all remaining actions which are due soon.
- The new 'Due or Flagged' option in the 1.6 Sneaky Peek gives you all remaining actions which are either flagged or have a due date assigned.
- Available gets you all of the Remaining actions which happen to be Available actions
- Next Action gets you all of the Remaining actions which happen to be Next Actions
Viewed this way, it makes complete sense that the sequential nature is ignored except for the Available/Next Action choices. From a usability point of view, you certainly want to know that some project actually has 6 (sequential) items due today rather than discovering that fact at 4:55 PM when you check off what you thought was the only one, right? What a nuisance it would be to have to check each project individually after seeing that there was a due item in the context view!