Returning from their 'home' game in San Diego in which the Raiders were favored by three points and won by exactly that, they take on the Colts at actual home where they are favored by 4 points. Here are the odds courtesy of OddsShark:
You would figure the Raiders would have been more favored against the cellar dwelling Chargers than they would be against a Colts squad still in the playoff chase. But a point in favorability is usually added for the home team. Plus division rivals often play each other tough.