fietsbob gave the generally correct answer to the question...most frames with chromed stays or rear triangles and lugs, especially vintage ones, are fully chromed under the paint; and forks with chrome socks are too. It is simpler to dip an entire frame or fork into the liquid baths to apply the electrical current than to dip just a rear triangle or forkends into the solution. And if one also wanted chrome lugs, they'd have to redo the process a 2nd time for the headtube/lugs section. The whole frame is dunked in the tank so you get chrome everywhere on it.
In most cases though - the chromed areas that are painted are not buffed/polished so that the paint will adhere better.
Of course there are more than a few exceptions to this generalization. For example - I cannot comment with certainty about all Cinelli Supercorsas...but my own SC Pista, which has chrome stays and lugs (not to mention very tender paint that has gotten nicked to hell) is not chromed under its color.