So I'm messing around with a new OU team on Pokemon Showdown, almost at 1200. I had just lost Staraptor to a Thundurus-I after I realized to late that it was a physical Thundurus with Defiant, not Prankster (and had used Intimidate and Defog). However I thought it was at low enough HP to KO with my Mega-Beedrill who had been sitting in the back, but I never got the chance to see because Thundurus outsped me, knocking me out w/ Wild Charge. I'm probably looking past something really simple, but I'm very confused as to how Beedrill did not outspeed Thundurus. Please help if you can :)
My Mega-Beedrill's speed stat is 427
Thundurus-I's max speed is 353
He was not Choice Scarfed, as he had used Knock Off the turn prior.
Trick Room/Tailwind was not active.
He was not holding the Quick Claw.
My moves were U-Turn, Poison Jab, Knock Off, and Drill Run.
His moves were Wild Charge, Knock Off, Hidden Power (prob Ice), and the fourth was unrevealed.
I won the battle overall, my Gardevoir outspeeding and KOing w/ Moonblast.