How accurate is GoogleRankings.com?

I was just doing some research on my competitors. I found that badminton-information.com appears as the 13th result on my Google search for "badminton", but GoogleRankings.com reckons that it holds position 95.

What's going on? Which should I believe? The former suggests a far more serious competitive threat than the latter.

It would be useful to understand this, because I've started to use GoogleRankings.com to record my SE rankings, so that I can present sponsors with records of sustained high rankings and demonstrate that my pages are rising through the rankings.

These data are very convenient -- I needn't sift through Google manually -- but I'm concerned that they may not be accurate.

(Note: Google has stopped issuing SOAP API keys. If you don't already have one, you can't use the GoogleRankings.com position tracking service.)