It ain't you babe. What you are seeing is a function of uneven shingle fade. All shingles fade, everything fades. It looks as if the shingles were installed properly, i.e., stepped and not ran straight up the roof. The issue is that shingles from different run lots were used. And they were not properly blended when being installed. It happens.
There is nothing at all that you could have done different. There is nothing that you can do to change it. If you followed the manufacturers cleaning instructions, any claims to be made would need to be made by the building owner to the shingle manufacturer under the terms of their warranty against fading. Good luck with that. The warranty claim will be denied based on the fact that the installer failed to follow the little note on the shingle bag to blend shingles from several different bundles when installing to avoid unattractive color patterning.
The shingles are now as clean as can be and as bare as they will ever appear. The starkness of the contrast will fade gradually because nothing exposed to the weather can maintain this state of laboratory clean for very long.