the idea of arming time i was led to believe was that you didn't want the shells to blow up in your face, so you added a mechanism to prevent the shell from blowing up in the barrel or when you were nearby. if you shot a shell into a ship where it stopped, lodged itself in a wooden timber or something and then when the arming time was over, exploded. the ensuing damage would then cause shatter damage back on your ship, as the flying spars and whatnot flew everywhere, including back at you, if you were too close. the mine launcher is of course not powerful enough to lodge the mine into the side of a warship, and it would do its unarmed damage as usual, but then bounce off into the intervening space, perhaps arming after the time limit is over, to deleterious effect, if the ships are very close together
rockets don't have to be fired from a shell and therefore don't need to have this precaution. the point blank vs long range distinction might be one of speed therefore, as the rocket thruster accelerates the missile it speeds up over time. the explosive payload might be unaffected, but shatter damage might be reduced as the actual impact speed of the rocket would be much less at close range than at distance.