As I understand it (although I have never seen confirmation by a Marriott spokesman), the Marriott folks credit the property that accepted the PP, BB, or EE at a slight discount to the "official" Marriott rate -- 80% was once mentioned -- so in a way the property would gain (through having to absorb less loss) if the rate they give you is lower than the Marriott rate.

As noted, the certificate says the conversion will be at the "hotel's current exchange rate" which many properties interpret as meaning their rate for converting cash at the desk. Arguably that rate isn't the proper one to apply, since the rate for cash is necessarily lower than the interbank conversion rate to account for the conversion charges levied by their bank (the PP, BB, and EE don't have to be converted by a bank and thus do not attract the conversion fee).

Other properties give you the rate on the Marriott currency converter program, and some will give you the rate as set out in the WSJ or Financial Times for the day. I like those properties.