Almost certainly the selection bias plays a major role - surgeries done early in a week are different than those scheduled later in the week.
I am willing to accept bias, but Why would Monday surgeries be statistically different from Wednesday? Clearly weekend surgeries might be different, but the effect is still present Monday-Thursday. I don't work in healthcare so there might be a selection bias that I am blind to, and I am curious if you have a bias in mind. Also they did track specific procedures and for all but one (with a low n) the procedures track…
I'm also not a surgeon...