Probably more likely that it's because Comcast has a wider sample, with more variety. Doesn't mean that they're worse or less reliable. They have options for lower packages, the people with the lower scores may be happy. Without knowing the package speed and the scenario the tests were taken under it would be hard to fully make an assessment like that.
I have improved charting that's on the way that will give you much more useful information when doing those comparisons.