The signal by the covering official is declaring the ball dead with a specific reason as to why (e.g. I'm declaring the ball dead in legal possession of a player in their opponent's end zone), whereas the signal by R to the press box is confirming that there is a score. It's the same signal, but has two different meanings in two different contexts. Incomplete pass, try no good, penalty declined, penalties offset, and coin toss option deferred to the second half all share the same signal as well.