The reason those 3157 SRCK bulbs work in your reverse sockets is because the reverse sockets are 3156 and have only 1 set of contacts, and the bulbs you bought have internal circuitry so that they are non polarized. So as long as the input signal, and ground are diagonal from each other in that particular configuration (11 o'clock and 5 o'clock), then you will get the 'high' output from the bulb. If these were switchback bulbs, instead of just white, you would have yellow reverse lights
As far as I know, there is no alternate version of 3156 socket with the contacts at 7 o'clock and 1 o'clock, but if there were, and you had them on your truck, the bulbs would be less bright, as that would give you the 'low' output for the parking light function. Hope that makes sense.