Sané did out pace Bellerin a couple of times, including the time he had his shot saved by Cech - partly due to the fact Bellerin switches off and is used to recovering with his pace, when he did the same against Sané he didn't get near recovering.
Also, KDB's massive error was not losing to Bellerin in a footrace, but walking inside and leaving the whole flank open so Cech's roll out to Bellerin was the obvious choice.
If he'd retreated to his correct position, Cech wouldn't have risked throwing the ball out in the first place, or at the very least would have had to think twice about it, and the counter wouldn't have happened.
It's not a huge error, but I think if there's an area of his game that's weak, I would say it's switching off at transitional moments. Once he's in defend mode he presses and harries very well, but it seems that every now and then he just switches off at the end of an attack and can get caught sleeping.