The real reason why results always vary?

Many experimenters are plagued by variation. Physicists cite Heisenberg for uncertainty in their measurements. However, most engineers prefer to blame their troubles on the devilish Murphy or elusive gremlins.

Those of us in the computer business swear by, or rather – at, bugs as the culprit for anything that does not go as planned:
“Things were going badly; …Finally, someone located the trouble spot and, using ordinary tweezers, removed the problem, a two-inch moth. From then on, when anything went wrong with a computer, we said it had bugs in it.”
-- Admiral Grace Murray Hopper, pioneer computer scientist

PS. The Admiral lived her life at ‘full steam’ as evidenced by this quote : “It's easier to ask forgiveness than it is to get permission.” (I advise extreme caution if you ever feel compelled to take action on this basis!)