Photography as "Mirrors" or "Windows"

I've been reading "Strange Hours: Photography, Memory, and the Lives of Artists" these last few weeks. I found an interesting thought from John Szarkowski:

What makes this more interesting for me is that my "Faribault County" work from 20 years ago would fit as a "window," but my more recent "4 a.m." work is definitely a "mirror." I don't know what that means, but it's made me stop and think about my own art making...