[This page](https://www.sealtest.ca/en/products/milks/1-milk) shows 250mL of milk having 45% of daily vitamin D. This is consistent with other brands like Neilson.
[Health Canada](https://www.canada.ca/en/health-canada/services/technical-documents-labelling-requirements/table-daily-values/nutrition-labelling.html) recommends 20 micrograms of vitamin D daily.
However, [a couple top results from a google search](https://www.google.com/search?q=how+much+vitamin+d+in+milk+canada&rlz=1CAOTWH_enCA845&sxsrf=AOaemvI7Q9cIDUNzoevlrRaaFikOHaYi3A%3A1642648227133&ei=o9LoYcPPB4KfptQPy_eD6A0&ved=0ahUKEwiDrcbArb_1AhWCj4kEHcv7AN0Q4dUDCA4&uact=5&oq=how+much+vitamin+d+in+milk+canada&gs_lcp=Cgdnd3Mtd2l6EAMyBQgAEIAEMgYIABAIEB46BAgAEEc6BggAEAcQHjoICAAQCBAHEB46BAgAEA1KBAhBGABKBAhGGABKBQhEGIEISgQIQxgDUMYUWO8kYJIoaABwA3gAgAH6AogB-guSAQcwLjguMC4xmAEAoAEByAEIwAEB&sclient=gws-wiz) indicate that at most there would be 3 micrograms in 250mL of Canadian milk.
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So hopefully the math is clear, 20 micrograms recommended daily by health Canada, but nutrition labels indicate 6 micrograms is 90% of daily intake.
Answer
Milk does not naturally contain Vitamin D, or at least it contains only a very small amount. It’s fortified with Vitamin D. In some countries, including Canada, it’s a legal requirement for milk to have a certain amount of Vitamin D added to it and it’s common practice for other countries to do the same.
So the amount of Vitamin D in your Milk is the min requirement + however much the company chose to put in it.