Directly from whoishostingthis.com faq page:

FAQ #1 What is the purpose of this tool? Aren’t there enough domain/hosting related tools out there already?
Actually, amongst the many web hosting/domaingin related tools we’ve encountered, we haven’t found one that gives you a clear answer to the simple question: Who is Hosting This Site? We’ve found that the best way to find good web hosting is to check out what your favorite web sites are using. This tool makes it very easy to do.
FAQ #2 Are these results accurate?
To our knowledge, there is no way to always return 100% results for requests such as this. However, we are using several methods to find out who is hosting a domain, and our results are accurate over 98% of the time.
FAQ #3 But, I’m quite sure the result is wrong!
There are several possible reasons for this. Most likely it’s one of these two: either the web site you requested changed their hosting provider *very* recently, or the hosting provider for this particular domain is just a reseller from a bigger hosting provider, and our result show the *real* provider instead of the reseller.
FAQ #6 I’m still sure the result is inaccurate!
As we’ve said, there is no 100% way to get accurate results. This tool is aimed to save you a lot of time you’d have to spend on research, but it’s not (nor it will ever be) perfect. However, if you’ve found an inaccurate result, tell us what it is and help us improve!
 
FAQ #7 I entered a website, and got that same website as the result! What gives?
This probably means that the owners of this website have bought an IP range and are self-hosted. It usually happens with huge sites like Google. Unfortunately, there is no way for us to give you more precise info. And to be frank the information shown is correct - they are hosting ther own sites. Just like other hosting companies host other sites.
FAQ #8 Is this just a scheme to boost some hosting provider?
Absolutely no. This tool is a product of an idea that came from two developers, Gordan Orlic and Stan. We chose the hosting provider which we were using before for other projects. No specific hosting provider is “behind this”, in fact, none of them had any idea we were launching this tool. If you see an ad for a hosting provider on this site, this is because this hosting provider paid us for an advertisement. In short: we are not biased in any way.