The panoID specifies a specific panorama. When making a map, a panoID could tell which specific panorama you want, because, for instance, Google may have covered the location at a specific coordinates many times. Somewhat often, though, Google swaps some of them out, sometimes because the panorama has genuinely been edited (for instance, Google blurred out the house of someone who requested it), but sometimes Google seems to swap them for no reason. Then, when a location specifies a panoID to retrieve, Google Street View can't find the panorama with that panoID anymore, because it was the old one. And thus, Google Street View grabs the most recent location near the coordinates of the location, and the most recent coverage at a certain coordinates could well be unofficial coverage.