360 Numbers don't add up!

[Monty]

@ Erik: I distinctly see a post above where people are taking the NPD data and adding in 15% to account for NA. If I'm wrong, what is the 15% for? Either way, vgcharts is adding in 15% off the NPD totals. Something I got poo-pooed for when I tried to go above 10%, and it was repeatedly beat into my head here that EVERYBODY knows this imaginary 10% rule. So, why the difference? Why is everybody else adding in another 5%, but at the same time ridiculing me when I said it had to be over 10%?