When you are in project mode, the sorting and grouping is by project, not by action, unlike context mode. Everything matching the view from a project is put in the earliest group that matches.
Here's an example which may illustrate what happens:
The earliest item due in test project 2 was due a month ago, so the whole project is put in the "Due within the last month" group. Beware of this with parallel projects!
For comparison, here's what happens in context mode:
(Sorry, I forgot to switch back to the standard theme before taking snapshots!)