Very interesting. Would you happen to know why I'm getting much better results by never using shampoo, just rinsing thoroughly with water, then using conditioner? For clarity, here's what happens:

When I'm using shampoo regularly, my scalp is itchy, I get dandruff, (my sister called it cradle cap), but my hair and scalp are otherwise clean.

When I'm not using shampoo regularly, my scalp is not itchy, no dandruff, it's all still clean.

I still use conditioner even without shampoo, because if I don't, it seems less clean, strangely.