It may have been the FIA mandated changes to the harvesting settings. I did not know until yesterday [*] that there are 3 harvesting allowances per race and that they change for every race. For Monza the teams can use a maximum of 7.5 MJ per lap for practice, 5.0 MJ for qualifying and 7.0 MJ for the race. So the energy available changes as the weekend goes on and the driving technique and the overall power available will change too.
Why? It seems the FIA don't want the cars to fall into "super-clipping" during qualifying because that makes them look poor on the straights. Then the power harvesting can go back to clipping/depletion Yo-Yo racing [**] for the first laps of the race.
[*] Thanks to Peter Windsor's excellent podcast.
[**] Other jokes are available.