Help me understand what I'm missing?

Som-ting-wong.
One start would be to pull all the rockers and check installed valve stem height for uniformity-both sides.
I use the 90 degree method for setting my preload but I question if it would be accurate enough for what
you are doing. The spread you are measuring speaks to an underlying problem that adjustable rockers would
just hide. My $00.02-------------------Good luck!