Your analysis is an oversimplification
Back then Weidman was undefeated. He was younger, hungrier, way more confident, and had less wear on his chin and body
Not every fighter ages at the same rate. Chris aged like milk. He always had average-at-best cardio, and once opponents cracked his chin he became an easy target
He was slow, even in his prime, but he made up for it with strenght, power, and good wrestling.
But back in the Silva fight he was just starting to peak, while Silva was starting to decline. You saw how Chael was able to take Silva to his limit. Silva was beginning to slow down, as he was already in his late 30s
Plus the Chael fights probably made Silva extra afraid of good wrestlers, and facing a young, undefeated jock wrestler like Weidman probably seemed like an intimidating prospect. Which is why he clowned around, hoping to lure Weidman into a striking battle instead of a grappling battle
Except he got too careless and Weidman found his button. You can call it a fluke if you want, but the circumstances were just right. I think Weidman would have won that battle 9/10 under those conditions