After a DEVASTATING loss to British YouTuber KSI in the big rematch this weekend, Logan Paul has some splaining to do ...

And explain, he does, in this video. "It's math, bro." Logan's pissed that the ref docked him two points for allegedly hitting KSI in the back of the head when he was down. It was the two points that made the difference and send Paul to the loser's corner. Anyway ... hear HIM explain it!

And check out the big names who attended the fight ... including Justin Bieber who walked Logan into the ring!