One cat's behavior towards the other

This may be a really strange question, but why does one of my cats lick the others bottom? It doesn't bother them in the least, but it totally grosses me out!!!

It's just natural mutual grooming behavior, some cats do it, some don't.
I've got two mutual groomers here and 4 that just don't much appreciate it.

TY for asking this.
When it comes to cleaning, Who is the dominant one?

Ben has been keeping very good care of Lucky's ears & bottom. Since the P/U surgery Lucky has been very unclean. It took a few weeks before Lucky would let Ben near that area. Ben tries to sniff/clean Molly's bottom but she won't let him.

Molly won't clean anyone

My two mutual groomers are pretty far down the totem pole.

Thanks for the info. It makes sense.