These are good questions but i think you can answer them for yourself with a little reflecting.
Spend some time analyzing your memory of these experiences and consider these questions:
1. Muscle fatigue. Was there a difference, however slight, in the direction of the force you were asked to resist? Your body will adapt to different vectors and you may only experience it as resistance, the "same" experience in all three exercices.
2. Neurological linkage and intent. Your eyes direct your intent and subtle muscular responses result in more efficient resistance.
3. Consider Newton's third law of motion: a force against an object results in an equal and opposite force. If you have a good base while resisting you are part of the floor and energy is reflected from the floor through a properly structured body back to the source.
Qi is as real as it needs to be. It's not wrong, but it is a cultural construct. We can experience the same kinds of things but the mind prejudiced toward science might construct different explanations.
Be well.
"Look, I'm only doing me job. I have to show you how to defend yourself against fresh fruit."
For it breeds great perfection, if the practise be harder then the use. Sir Francis Bacon
the world has a surplus of self centered sh1twh0res, so anyone who extends compassion to a stranger with sincerity is alright in my book. also people who fondle road kill. those guys is ok too. GunnedDownAtrocity