The problem is that the cars were never at the same fuel loads at the same time out on track. The run ends earlier for Ferrari, and even earlier for Mercedes (because they were on different programmes).
This is a snippet from the point when laptimes start to get comparable, until the end (before this, we have big fluctuations with teams testing different things) :
Ignore what seem to be some weird timing bug laps for Russell
And if you go by the fact that Oscar finished the run last, with Leclerc finishing it second, and Russell finishing first, applying a fuel delta, Merc actually seems to come out on top when it comes to race pace, with Leclerc right behind them and with McLaren being around 0.4-0.5s further back.
But the thing is, you can never know with what sort of fuel levels they started these runs, how much is the driver being asked to push, what engine modes they are running. Nobody knows really. The good thing is that we only have to wait a week before finding out.