OK. Here's a laymans explanation. I hope it helps.
A full HD TV has 1920 pixels across and 1080 down. So, when one has a Blu-ray player connected to such a TV playing a 1080p Blu-ray movie, with no picture overscan, just 1.1 pixel mapping, what is happening?
What's happening is that each individual pixel is being instructed, it's being told exactly what to do, ie, what colour to emit and for how long.
These instructions if you will, are contained on the Blu-ray disc, they do not change, ever. They are exact. No Blu-ray player can change them.
So if you use different BDP's to play the same 1080p Blu-ray movie on the same TV, the results will always be the same, each pixel will always be told to do the same thing for the same amount of time, every single time.
This is why no BDP can have any effect on 1080p image quality.
To be smoother with motion they would need to give different instructions.
To be less grainy, they would need to give different instructions.
To show deeper blacks, they would need to give different instructions.
BDP's are not intelligent, they pass on the instructions from the discs.
These instructions never change, and this is why 1080p image quality is exactly the same between BDP's.
Differences are simply not possible.