Why does Google only show the review stars for some listings in the Places results

[joshualfink]

I have a client who shows up in the Places results for his targeted keyword phrase, but one of his competitors' listing shows 5 gold stars next to the reviews which causes him to stand out visually. I was under the impression that at one time the stars only showed for listings that had official Google reviews, but the other listings in the results also have "Google Reviews" but no stars.

[ResslerMotors]

How many reviews does each company have?  My gold stars didn't pop up until I had 10 reviews - half being Google reviews.