I ride with two different women or we catch up on our morning rides as we are normally out there about the same time. They ride slower than me on average but keep a nice but not tantalizing pace we generally converse at.
I like to ride with them for the view.
OP...this riding at a given speed applies to every rider on the road. From pros on down. Horner was used as an example and even though he is an unbelievable rider, he admits to not having the strength of many riders in the tour. Even with a A or B group ride there can be a wide variation in power...some guys just try to hang on and others like to pull.
Also, there are a few women riders on the road that I ride with who can keep a 20 mph pace mile after mile on the flats and can pace line much faster.
I think the point is try to be respectful of others which shouldn't be hard because every once in a while each one of us is humbled by a truly strong rider.