"Bowl two games, get one free"

How would you interpret that offer?  The coupon above is available on the web and applies to a local establishment here in Madison.  Sometimes when I take the coupon with me and bowl six games, the clerk at the counter will charge me for four, interpreting it to mean that for every two games I bowl I get a third one free.

On other days, with a different person at the counter, when I bowl six games, I'm only charged for three (because for every two games I bowl, one of them should be free).

The coupon can be interpreted both ways, because what it says is "bowl two, get one free."  If it said "pay for two, get one free" then it would pretty clearly be every third game free.  Curiously, it's the owner of the establishment who takes the more liberal interpretation of half the games free.

An excellent, clean, well-run bowling center, btw...