No need to speculate, they explain how this works right on the main page of http://pizzaforcoins.com :
"We set our exchange rate APPROXIMATELY $0.50 Cents less
then the current Mt.Gox Rate. The reason for this is that
with rapid fluctuation of rates and occasional drops, we
do not want to be left holding a worthless bag if the rate
drops for some reason. We update prices on a daily basis
to reflect this 50Cent rule."
"We set our exchange rate APPROXIMATELY $0.50 Cents less then the current Mt.Gox Rate. The reason for this is that with rapid fluctuation of rates and occasional drops, we do not want to be left holding a worthless bag if the rate drops for some reason. We update prices on a daily basis to reflect this 50Cent rule."