Which data did you use? Since the leagues were from the Quarterfinals and onwards, was it data from the group stages, euro qualifiers? Averaging the last 5 games would introduce a lot of variance, especially in tournament football.
I was using data from the group stages. And I agree that just using the last 5 games had a lot of variance, but unfortunately, I didn't have data from before the Euro.
This approach would be interesting to use in league football (La Liga, Premier League, etc) as there is a lot more data available across different seasons.