M.M.
Because she's 2, just learning language, and exercising her independence all at the same time. This is a very big age/time for little people.
My son is 2 and does the same thing with various items.
She'll get it. Just give her some time. Since you know that she knows the difference between milk and juice, I wouldn't worry. Just keep repeating what it is that you're giving her, and eventually she'll come around.