Let's break down the Polygon score using scientific methods and how data is typically handled in scientific study.
Quick math lesson - 95% of all numbers in a data set fall within 2 standard deviations from the mean. Scores/Data that fall outside 2 standard deviations from the mean are typically deemed "outliers" because there is less than a 5% chance that it is a "true" data point. This is why in any scientific paper the "gold standard" for proving that something is likely to be true is to look for a p-value of less than 0.05 (i.e. you can say with 95%+ confidence that your conclusions are accurate based on the data).
Using the scores at the top of this page, including the Polygon score, gives us the following:
Total Scores: 13
Mean: 7.769
Std Deviation: 1.179
Gies score lies 2.35 standard deviations away from the mean, meaning that there is less than a 5% chance that his score is a "valid" piece of data.
If Metacritic was smart, it would use this method, throw out all scores more than 2 standard deviations from the mean (on both the high and low end) and use the mean of the remaining data that as the Metacritic score. If we do that here, it rejects Gies score of 5 and gives a mean of 8/10, which seems about right. I won't quibble if it's really a 7 or 9 out of 10, but certainly not a 5.