To understand how this works, you have understand how sebum production is regulated – in other words what turns the sebum glands on and off. It turns out that it’s the presence of sebum on the surface of skin that controls sebum production. How is that possible? Well, if you REALLY want to know, go read this study in which scientists stripped oil off skin and then measured how long it took the skin to re-oil itself. Their data indicates that the presence of oil on the skin’s surface sends a signal to the sebaceous glands to turn off. This signal is caused by either the pressure of the oil in the follicle or by the creation of a chemical signal that travels back down through the skin.
So it looks like it’s the amount of oil on the skin and not the degree of dehydration that determines how much oil the glands produce.