Line under taskbar icons

There's a weird line below some taskbar icons. Before, I knew that line indicates if app is open/focused, but there's a line under closed apps now, for example third icon in my screenshot, while second icon is not underlined! And why is so longer - each line has a different width like it indicates something, but no clue what.

How to remove the line under apps which are not running?

What does the line means even? Why its so inconsistent and confusing?