The editing in ESB is pretty much perfect. The troopers AREN'T approaching from the same entrance Han and co appear. If you check the whole sequence again you will see that the snowtroopers come in from the left on the hangar ( where the snowspeeders were originally parked), whereas Han & co come in from the right ( where the x-wings were originally parked). How can you tell this? Well look at the position of the falcon in the wide shot where Han & co are running towards the Falcon. The Falcon isn't angled straight on . But, when the troops appear, as in the photo above, then it appears to be at 90 degrees to the troopers. So, from that, we can determine that they are coming in from an entrance to the left and that is where they set up the gun