Read more carefully. It's never been said that there aren't any unbiased voters. There are, but if something is worth a high score, those unbiased voters would be overshadowed or outnumbered by others.
But you implied it when you said reviews tell the truth better than an individual's opinion. Reviews aren't always reliable and shouldn't be used to determine whether you should read or watch the literary work or movie.
Dragon Ball Z getting a higher rating than Naruto does not make it better than Naruto. It's all subjective, and the high scores aren't always accurate. I've seen how people rave about Al Pacino's Scarface and give good reviews, but I think it's an overrated film that's at best mediocre. Those same people who praise Scarface probably give Godfather a lower rating or below 5 because it doesn't suit their enjoyment - lack of constant cursing, violence, and blood even though Godfather I and II are easily superior.
Reviews don't really tell much, IMO, unless there's actually a well thought out review. Regardless of what anyone or any review says, for me Naruto is better than Dragon Ball Z, at least Part I Naruto. Part I Naruto is also better than HxH, and I don't understand why people say Naruto took inspiration from HxH when they're so different.
Sadly, Kishi took away complexity and development and turned it into another typical shounen like DBZ and HxH.
You have to define "better". Better as in healthier food or better in taste.
But regardless, so if you see a score of something 2/10 let's say, you won't trust it and you'll watch it to see for yourself? Man, that must be exhausting and disappointing.
Why not? People have different tastes in manga. Those who prefer Part I enjoy a more complex story telling where the fights aren't so identical to each other whereas those who prefer Dragon Ball Z enjoy mainly fighting. Even score of 2 out of 10 may not be entirely fair, because the work itself could be of high quality but the complexity or message of the story doesn't sit well with most people.