I think the general idea is that the stats labs use to decide what is normal is not necessarily ideal, and like everything else will vary per person.

Too much Vitamin D3 can be toxic, so doctors are reluctant to tell you to go over published guidelines. but if you don't have other issues, and stick to what you can get OTC from a normal vitamin brand and follow the instructions, you're probably safe. There was a story a while ago about a woman who died from too much D3, but it turned out she was taking a whole bottle's worth every day for years.