When clicking a track, the track number goes blue. 
When using arrows, the track name is highlighted, but the original track number stays blue.
 
Is there a difference in the selection mechanism?  Why does the original track number stay highlighted when a new track is selected with arrows?

It depends on where the track strip is clicked. By clicking on the track number, the track is selected. Clicking elsewhere on the track strip or using the arrow keys changes the track focus. Changing track selection changes focus. Changing focus does not alter track selection.

scook
...Changing track selection changes focus. Changing focus does not alter track selection.

^^^^^^^^^
This is a key difference...