Whole chicken roasting time
The shops are likely going out of their way not to get sued by sticking to safety guidelines rigidly. The packet instructions usually result in a dry and overcooked bird. The various other times and temperatures are all attempts to get a safely cooked, attractively browned bird without overcooking.

However, roasting meat based on oven temperature and the weight of the joint is the wrong way to go about things, because ovens are rarely calibrated correctly (the temperature is often wildly at variance to that set on the dial) and the weight of the meat is rarely nicely rounded.

If you really want to cook any meat (and many other foods) properly, you'll invest in a digital probe thermometer. Then you can simply cook the meat until it reaches a safe internal temperature and no more. They aren't expensive, you can get them from Amazon, and once you've got one you won't know how you did without it.