It seems to me that removed from the game means just that; permanent removal from play, while removed from map returns them to the Pending area just as if they would at the end of a turn of Tactical side.
If there is a difference, in which cases are they removed from the game as opposed to returned to Pending?
Yes there is a difference.
Page 7 itself gives you situations where they're removed from play (not just from the map): Capture by enemy unit, and sniper activity. See also 21.12c & 21.12d.
Edit: Ninjaed by Chad!