maybe it is all down to stratergy. some circuits take more out of the engine and some circuit are more reliant than power. maybe different engine builders have been more conservative than another. Plus the engines don't have to be used concurently. there is nothing to stop someone using there 8 engines in the first 8 races and then going back and revisiting them later in the season
Maybe, but it depends on the maths. If there were ten races left, and the Mercedes teams had two new engines left, and the Ferrari engined teams three engines left, then it's possible that the Mercedes teams might be in a better position as their used engines might be less used than the Ferrari ones.
But with four races left, Mercedes teams having one fresh engine left, and the Ferrari teams having two fresh engines left, there is no possible combination of engine use by Mercedes which can match the average "engine freshness" that can be achieved by the Ferrari engined teams. As detailed above.