Well the numbering system in it is valid. Do you mean that it is being spoofed? Perhaps the number isn't assigned to anyone right now but it could be in the future. How is an independent website like 800notes supposed to keep track of each and every number that goes in and out of service?
How does Mr. Number and Number Guru do it? I don't know, but if they find a number is 'not in existence' they report it as so. Maybe they have someone assigned to dial the numbers that are reported to them, and then document what they find if it comes back as 'cannot be completed as dialed.'
Saves us from having to call it to find that out, which is dangerous in this day and age.