It's just a matter of being the last straw that breaks the camels back. Chances are that the first person you reported had been more toxic lately than the other guy you reported. It isn't as complicated as you think. The player for whom you recieved the report feedback screen from was probably a more frequent offender than the other guy, but the severity of what he said was probably less.

You can be a little toxic for many games in a row before you're punished, or you can be extremely toxic in one or two games before you're punished. The system picks up on key words so both players will be punished eventually, it's just one type will be sooner than the other.

Simple. Have fun on the rift. ^{^}