The standard lab ranges are defined by “at least some people don’t show symptoms of deficiencies or issues or etc at these levels,” not “everyone is guaranteed to be fine in these ranges.” I was “in range” with B12 until I started developing nerve damage from a deficiency of it, with years of symptoms leading up to it that were ignored because I was “in range.” I was “in range” with folate, because the standard lab tests inactive folic acid and not active folate. I was “in range” with ferritin, and yet had significant improvement from taking more iron. I was “in range” with potassium and then ended up in the ER multiple times from hypokalaemia so bad that I could barely move.

You get the idea. “In range” is not the same as “will get no benefit from supplementing it,” but it’s the kind of thing that ideally is done with proper medical assistance if you think you’re deficient in things.