Google doesn't sell their list to you. They give it to you for free. Using their list costs them money. Pumping up numbers gains them nothing but the headache of PR issues when they get a false positive.

Spyware filters used to boast about how many domains they filter out because they wanted you to buy their filters instead of someone else's. By the time they hit a false positive, they've already sold a year's subscription to that customer.

The incentives are different.

Google wants you to use it. If it blacklists excess domains that hold legitimate sites, their product gets worse. If they blacklist illegitimate sites, their product gets better.

Nobody sees Google's numbers except Google... in other words, the numbers are not a sales tool for Google like they are for anti-virus/blocking companies. So, there's no reason for Google to pump up their numbers, it would just be extra work to make their product worse which wouldn't make sense.

Nothing, but they haven't done it so far, and they don't really have any incentive to do so.

It doesn't really matter that it's Google. It could have been Microsoft, or PAN, or McAfee or some fly-by-night vendor. The problem was Radix taking the list as iron-clad truth and disabling the domain without any notification or way to resolve the issue.