Marriott has now introduced a minimum of $1 difference that needs to be found when you find a lower rate elsewhere to be acceptable. From the T&Cs:
Marriott may deny claims where the difference between the comparison rate and the rate on the Marriott reservation channel is less than $1.
This is certainly a change (not sure how recent) as I had a difference of a cent accepted previously - but I still think this is somewhat reasonable...
Greetings - Dirk