Selection for batch operation in Gmail's new material design

I'm confused, while I'm trying to understand the new Gmail "compact" style user interface. This is the view, where left to the email's 2-line representation a checkbox can be found. I can't really grasp the UI presentation of "selection". Please, take a look at these four screenshots. 1. **Selected and checked** > 2. **Selected but unchecked** > 3. **Checked but unselected?** > 4. **Selected and unchecked, but different from (2)** > It is clear (from the operation appearing at the upper part of the screen), that 1, 2 and 4 are selections. Why is the UI different in these cases? Also clear, that 3 is **not** selection... but a checkbox has a mark in it, and it normally *used to* mean selection for further operation. So I'm totally confused, what the UI tries to tell me. Could you please shed some light on what - checkbox (marked and unmarked state) - colored background - thick, and thin frame of the checkbox mean in the context of email selection?