Originally Posted by tdkkart
Pushrod lenghth should have no affect whatsoever on the lift at the valve, except on extreme examples where pushrod flex is a problem.
What I would suspect is that your rocker ratio is not exactly 1.5, woldn't be unusual at all, especially with "off-brand" rockers. Rocker flex is also a possiblity if your springs are really heavy.
Set up a test stand to check your rockers with very little spring pressure, I'll bet the ratio is actually closer to 1.44 as indicated by your measurements.
Bingo, we have the correct answer
I've known guys to make friends with the parts counterman at the local Chevy dealer so they could take a set of rockers home, check the ratio and return for another load of rockers until they find the highest ratio for a complete set.