Thank-you! because I thought of that too. I'm checking these rockers without lifter on lobe.
I removed the rocker shaft already and checked the lengths of a couple of pushrods and they all the same, and not bent.
It seems, by process of elimination, the only thing it could be is slightly bent or worn rocker shaft or the rockers themselves. It just seems so weird, I figured there must be something I'm missing here.
But the original motor was well worn, with a spun bearing, and the rockers are about the only thing that haven't been rebuilt or replaced. I just cleaned them.