Sometimes people who restore pipes merely cover up oxidation with oils. And, after your lips and saliva removes the oils, you see the oxidation. This is why I am not a fan of those stem oils. Not that they don't work to make the stem look shiny, but that so many folks use them to cover oxidation or to make an unpolished pipe look polished. It is also a way to cheaply and quickly make a pipe look appealing to a buyer.
Also, body chemistry. Your saliva may be very low PH making your stem oxidize. So many variables.