I took a closer look at the ratings and on ralphmaltby website it says, "In short, the higher the Playability Factor value generated by the formula, the more playable and forgiving the particular design should be."
A higher rating doesn't necessarily mean a better club, it just means it's more forgiving. I wouldn't choose a club just because it is more forgiving than others because there's a lot more that goes into which irons go into my bag. For example, top line looks, sole interaction, shot-shaping abilities, feel at impact, etc. or could overrule how forgiving the club is. Maybe others feel the same way.
I don't know about the bias, but I thought the results were interesting. Attached it for others who want to check it out. Looks like it's dated 05/2016.
MPFRatingsChart.pdf