Thanks for reply ckolivas. Is this uncertainty you refer to because nobody knows for sure how sha256 actually behaves? So if a miner were to work that out it is possible they could select contents to increase chance of finding a solution, but if they don't they can't? Pls feel free to point me somewhere else so i can educate myself.
People do know how SHA256 behaves. What we don't know is if someone out there has discovered a way to shortcut/cheat the SHA256 process without the rest of the world finding out.