Nothing in SEO is 100% - it's barely 50%. The entire industry of optimizing for search is a shot in the dark - and all we have are educated guesses at what works, since Google and other search engines largely keep their ranking methods a secret (even to their own employees) and they're changing constantly, so there's no surefire way to game the engine.
No tool can deliver 100% data - because it's ultimately all based on Google's keyword data and trends, which they have admittedly obscured, because they want people to spend money on ads, not on free SEO.
All these tools like Moz are doing is capitalizing on making it easy for people to do initial research - but they also created a grading score because people have no way of telling if what they're doing is working, so at least now they can get a grade and feel like it's making progress, even if they're getting 0 results from it. It's a smart move by Moz - especially once "pagerank died" - although PR never really died, they just stopped pushing live updates to the number.
