This is my assumption only. I think Google does rank their own website based on similar algo like other common websites. However, most of Google services are likely to get a huge amounf ot important links automatically. Especially when those service was in a first launched. Both from Google itself and other authoritative site such as CNN / Technorati / Techcrunch / Wikipedia / NYT / etc. That's why most of Google services site has a good rank in many keyword terms.