You say the game got mostly 6's and 7's. Simply not true. The game has 5 reviews in the 60-70 range. 10 reviews in the 80-90 range. That's not counting the ones above 90. More 8s than 6's. Probably why the average is closer to 8 than 6.
Just a quick way to show the strange bias in this post. It's a Kirby game that's a bit less good than the others in the series, and the score is also a bit less good. Nothing more than that.