It's based on the opinion of those 20 men mentioned in the article. I think they are mostly Calcio dudes so it makes sense they would be a bit biased towards the Italians, but it's still impressive that Allegri has the number 1 spot. There isn't such thing as unbiased opinion anyways, we are all biased in one way or another.
And I'm pretty sure they were given those ten names and only had to but them in their preferred order, thus some have included an unofficial number 11 in their rankings.