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is this just an extrapolation or does my body actually constantly produce the same amount of vitamin D if i sit in the sun even if I don't have a deficit?

Because usually phisiological processes are in some kind of equilibrium and regulated, so it's not straightforward to compare it to supplements.

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7-dehydrocholesterol is photolyzed by UV to previtamin D. There is then spontaneous isomerization between this previtamin D, vitamin D and 2 inactive isomers. As vitamin D is used by the body, and the inactive isomers remain in the skin, there is an equilibrium - as vitamin D is used by the body it is drawn from the skin, leading to more conversion of inactive isomers to active vitamin D.

The article goes into some detail on this, my understanding is that Vitamin D is produced in inactive form with sunlight exposure and then this is more-slowly released as needed (over winter apparently, but nobody's quite sure apparently).

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