There are lots of ways the poll could be slanted.
For example, when Azzarello started his run there were 90 000 people buying the book. On the polls launched at that time, there were hundreds of people voting, largely in the positive.
Now, we have a fraction of that number voting. The CBR reboot no doubt plays a part in that but also its just reader drop off. So its possible that many folks who gave it five stars a couple of years ago are no longer reading it. Similarly others who rated it low may have changed their minds, though based on my experience with comics and fans that seems a less common trend.
Is this data definitive. No. And of course the appreciation of the material is itself subjective. But I think its a better way of gauging peoples feelings on the book than just comparing it to the other New52 publications.