Mumsnet don't monitor all threads. They only delete stuff that gets reported. It appears that some of mnhq staff delete more than others. Some staff seen to take the time to check things out and some just delete everything requested.
I've had posts deleted for spurious reasons before now. Sometimes I'll discuss why with mnhq, but most of the time as they don't tell people, most of the time, when a post has been deleted I can't even remember what was written on it.
I've had a post deleted before now because i said a member of parliament looked like he was in rock band that played out of his parents garage!
Hell I've just had a gif of three dancing potatoes deleted!
And yes some threads get monitored and reported so many times by people who don't like what we say that they end up with a member of mnhq watching the thread in real time.