Funnily enough I only just saw a documentary recently that explained this, apparently there was a whole scene that never made it past editing where it shows the rebels using a different method to get into the bunker.
In the scene where the biker scout runs around the corner to be surprised by Han and the rest you'll see the bearded rebel put his gun down, in the missing footage he puts it down to change into the biker scouts armour and gain entry into the bunker in disguise. Hence you see the remnant of that missing footage when you see him with his arms up surrendering in the one you mentioned.
**** I'm a geek.

:lol:
Check out this doco on youtube, it explains it all much better than i just did. Be warned it's seven hours long!
http://www.youtube.com/watch?v=jeiL3Eru1A0