3 MGM properties raising resort fee

I've just finished a comped stay at Bellagio (3 nights) & then 4 comped nights at Mirage. I was expecting to pay resort fees, however these were comped also. I'm gold tier at m life. Although I get to gold in about a week.

So I'm not sure how this works.

Maybe your play while onsite enabled them to comp the resort fee.  My play probably won't reach comp level, and I'm surprised I even had 4 nights comp anyway.