It is, on average, in the United States.
Polygon can make whatever review scale they want, and they have as it's clearly there for everyone to read, but that's not going to magically break cultural perceptions of numerical scores.
In the US public education system, a 6.5 or 65% is considered "Below average", even if that may not translate 1:1 to Polygon's scale.
So, when I see a 6.5, I think "shit" and "not worth anyone's time", even if that's not really fair to Polygon. Can't help it, I was literally raised that way. A 5/10 is not average in my mind, a 7.5/10 is. Most American game review sites go by a 7.5=average scale. Polygon chooses not to. So, scores are going to feel dissonant sometimes.