Why does my Google Goggles quality score keep changing?

[having made no further changes]

I optimized one of my websites using Google Goggles and went from a Poor to an OK/Great score, much to my (and Google Adwords) delight.

Just for fun I then tested the website again the next day, having made no further changes, and suddenly my quality score was back to Poor again.

How can this be?

Fortunately Adwords are still showing my keywords as OK and Great so they don't seem to be as fickle as Goggles!

This made me curious and I Goggled another website several times, and the quality score changed quite dramatically. How can the capture form rating  go from Great to Poor when absolutely no changes have been made to the page? How can the number of links score go from Great to OK when no changes have been made to the page?