Just a hypothesis:
If a publisher is paying 50 people to write a favorable review about the newest Grisham novel, they write the review, post it and then move on.
If an indie author is getting friends and relatives to write a favorable review and post it, they are more likely to check back and give "unhelpful" votes to reviews that are more critical of the work.
For those of us who are higher up in the Amazon rankings, a bunch of negative votes can noticeably affect our position in the rankings. It's happening to me right now because someone is gaming one of the products I reviewed and is making sure that any reviews of less than five stars are getting multiple negative votes. I've already contacted Amazon about that.
Perhaps something like that was happening with particular indie author reviews, someone complained, and Amazon was able to isolate the reviewer who was trying to vote down other reviews in order to make theirs more favorable. The lower number of reviews for an indie author would make the reviews easier to police and easier to figure out who is trying to play the system.