Selected answers from the Dumb SEO Questions G+ community.
Alan Bleiweiss: because no tool provides entirely accurate or consistent data. None. Not even Google Analytics.
Here`s the more important concept though: Moz uses a proprietary algorithm based on only somewhat reasonably valid methods .
Where it falls apart is Moz has NO WAY of knowing which links from which domains are entirely ignored by Google, and which links from which domains are considered toxic. So Moz, like all 3rd party tools, can only evaluate signals from their blindfolded understanding of link signals.
And that`s made worse by the fact that Moz doesn`t have a complete database of domains on the web, so that too is another blind spot. And SEO is much more complex than any third party tool data point. I have had countless clients with higher ranking visibility in spite of having a lower Moz DA score than their competitors.
So it`s better to look at what the variety of industry tools show you as far as known issues, and apply knowledge you gain directly about SEO ranking factors, to look for ways to continually and consistently make the site better and stronger. That`s a much healthier way to expend energy.
Francois-Pierre Marcil: Look at Majestic and Link Research Tools, both are a lot more serious than Moz, SemRush, Arefs in terms of accuracy. Yet, as Alan says, none of these tools are very accurate.
I`ll add another layer to what Alan says: Google is not accurate either. Their own evaluations vary all the time as the web is a big place and even they can`t really do more than get samples. So as I often say: every minute spent worrying about link metrics is a wasted content production minute (and this is what makes winners vs. losers).
Michael Martinez: No third party SEO metric provides any insight into what major search engines like Bing and Google have indexed, allow to pass value, or ignore.