Playing around with the RAM to manipulate the scroll, I found that the last level (horse autoscroll) has a fix scroll count of 8. It means after the game scrolls for 8 full screens, the level automatically ends.

If we ignore all the lags, regardless Asteris's position on the screen, the cutscene will start at the same moment.

But, this is an in game cutscene where Asterix runs to the right and obelix enter the screen to run with him. If we look at the moment when the screen fully fade too black, the time would be different depending on where you are on the screen before the start of the cutscene.

Using an emulator (Bizhawk) I counted the frames between the moment Asterix start to jump out of the chariot and the screen turns all black.

being on the very far right, you would save approximately one second. Once the background turn black, it seems to be a constant 48 frames.

I'm not saying we must update the game rule but it would make sense to stop the timer when the next screen shows up (obelix smashes the colosseum), or when the final screen shows up (everyone around a table at night).

Some games stop the timer at the final screen, for example TAZ on SMS. In my opinion it's a way to say "I've beaten the game"