Me and my brothers partners managed to have a completely different experience of our Mum, despite her treating them exactly the same. And it was entirely down to me and my brother.
I'd ask my Mum for advice, then go away and consider that advice, decide for myself whether it was good advice, and then present my thinking to DP as my own.
My brother on the other hand, would ask Mum for advice, and then go back to his wife and say "Mum says we need to do this...."
As a result, to SIL, my Mum was an interfering busy body, whereas to my DP, she was a lovely laid back woman who she got on famously with.
This rather mystified DP and SIL, how they could find her so different, until one day I caught my brother in the act. We'd both been thinking of getting a new car, and chatting to my Mum about it. She suggested buying one a year old rather than brand new, as it wouldn't depreciate as fast. I filed it away that it was probably a good point, and to do some googling later. The four of us were in the pub sans Mum and my brother out of nowhere comes out with "Mum says we have to buy an old car rather than a new one"
I'm like "Whoa there a minute" and pull him up on it, explain to SIL that we'd been chatting about it and that my Mum had been making a suggestion, rather than an actual decree.
SIL and Mum had a chat, my brother got a bollocking from all sides, and my Mum and SILs relationship was much more cordial after that.