4 on 4 vs 5 on 5 (penalties)?

I know all of the rules except this one. when I'm watching a game and theres 2 players that commit penalties at the same time, sometimes it results in a 4 on 4 for 2 min but sometimes it stays 5 on 5 and the players go to the box. for instance in the buff phi game hartnell and gaustad each got roughing minors and it resulted in 4 on 4 but I've seen instances where it stays 5 on 5. can anyone help me understand which infractions result in which occurrence?