The only way that those overall percentage makes sense is if you have experience ranking terms at different difficulties. At least that's what I've found. It's generally accurate but void of any helpful context in and of itself. The more meaningful data is the underlying metrics of the pages competing, specifically:
- Domain & page strength
- Number of in-bound links with matching anchor text
- How many times the pages have been liked, shared, +1'd and tweeted
- Unique C-Block IP's linking to the page
- On-page keyword targeting.
Those factors are what the overall percentage is trying to convey, but it's generally just good to use as a starting point -- SEO is meaningless without context!