My experience is that 99/100 matches there is at least one red bar player.
Also from my experience i can say that one of those red bars is the host.
And the host has red bar, because everybody is playing on his connection.
Probably he has a very good ping, but too poor bandwidth for all players to play on. That's why red bars are mostly MVP's- they are hosting, so nr. 1 connection which counts.
This is the reason that after 2 years there are still red bars, every match......Bungie won't ban them, because they know it's P2P's matchmaking-system's fault.
Everybody knows that when you have very bad internet (red bar), you can't make a kill. Have tried it myself.
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The only time when everyone is full green bar is if players live in such close proximity towards each other that being host barely affects anything. With SBMM being so tight at the moment, full green bar lobbies are impossible unless in the lowest tiers of the algorithm.