Coincidental vs. Off-setting penalties- Why?

[DVRichard]

I have been playing and/or coaching hockey for over 40 years and I never gave this much thought but for some reason, I am now very curious as to why there is a difference between off-setting and coincidental penalties. I know that off-setting penalties means they play 5 on 5 and coincidental penalties means they play 4 on 4.I also know (roughly) how that is determined. If player A from the blue team elbows player B from the green team and player B from the green team turns and slashes player A from the blue team, as retaliation, these are off setting penalties. But if player A (Blue team) elbows player B (green team) and player C from the green team jumps in and roughs up player A as retaliation, these are then considered coincidental. (if I understand correctly) It is still one player from each team going to the box for 2 min.

My question is WHY is one a 4 on 4 situation and the other a 5 on 5 situation?? Either way, the two teams have equal numbers of players on the ice and the two players are coming out of the box at the same time so WHY is there a difference?

Basically, I'm just looking for the logic behind the difference? Your help would be appreciated!