[Antilochos]

Just found this on MotorSport, explains how George was able to catch Valtteri.

Bottas said after qualifying that he was struggling on corner-entry, marking a new issue to that encountered in Bahrain, which Mercedes later related to tyre warm-up. The problem was even more noticeable in the race as Bottas struggled to get his tyres into the right temperature window in the cool conditions, leaving him stuck behind Lance Stroll in the first stint. 

George Russell was able to gain time on the Finn by virtue of his warmer slick tyres - fitted two laps earlier than Bottas's own mediums - before their race-ending crash on the approach to Turn 2.

It marks a change in fortune for Mercedes. The team has traditionally fared better in cooler conditions and struggled more in the heat with its recent cars. Now, it has the opposite issue.

Interesting.
But they also had a hard time with tyre degradation (still thé reason why Red Bull won second Silverstone race last year) and there seems to be a big improvement. Could those two things be connected?