i assume you lift it up to make adjustments? when you lower it onto the blocks the suspension isn't settled so won't give a proper reading. you need to roll it back and forth a couple of times to do that. the only way to do it as you are is with 'grease' plates on top of the wood stands.
neil.
Yes you are spot on correct. I have greased plates for front tires to rest on and tire slides out a lot once I bounce suspension up and down. Also verifying ride height before measurements.