I don't remember when Marriott.com changed but it used to show you "Rates starting at..." So it's quite likely you saw the $189/night but that was the rate for just one of the nights of your stay. The other night was a much higher rate because, as you said, the hotel had a convention going on so it was quite full. In that situation it was demanding a higher premium for the rooms. Now Marriott.com will show you the average rate for all nights of your stay which is a far more user-friendly approach IMO.

My gut tells me you actually booked the higher rate for one night and the $189 for the other night. Others are giving you good words of caution as well. Will Marriott close your account? Unlikely. But if you create a pattern of this your account becomes an easy target.

-RM