I like Misko's answer. (Five posts back, now.)
Imagine two boys from Tattooine (or any other forceforsaken planet where it's a hard life). Each wants to get out of the sand-dunes(/jungle/snowdrifts/whatever) and decides to join up. One joins the Imperial forces, the other finds themselves with the Rebellion. Not being good enough to bullseye womp-rats from a T-16, they find themselves in the respective infantry units, rather than jockeying Tie Fighters or X-Wings.
Then, one fateful day, their paths cross again. Our Imperium lad finds he's staring at the Rebel lad he grew up with back home on whatever rock/forest-world/iceball it was. Training, and even mental conditioning are cut through by the sight of his best friend, wearing the typical Rebel attire, directly in front of him. The Rebel lad looks back to see... yet another stormtrooper aiming his gun in vaguely his direction. Bang, bang! Squeel, squeel!? Ah, but alas, only one shot struck home. And the tragedy is that the only person who knew of the connection, and of the possibility that under different circumstances this would be a joyous reunion, has just been cut down...
So many Galactic Standards (or whatever they use, again) in Imperium taxes have been used to train the Imperial recruit and mould him and equip him with the latest in weaponry and armour[1] and send him out in the field. Meanwhile the Rebellion spends its funds (from wherever they get them) hacking into the Personnel Databases of the Imperium (hey, a single R2 droid can break through into the computer systems of an entire Death Star station!), doing a smart match of where individual troops are deployed against the backgrounds of their own ranks (unbeknownst to the ground-pounders themselves) in order to deploy their own forces in the most optimal way to ensure a 1-to-1 match of Rebel soldier to Stormtrooper in as many firefight engagements as logistically possible.
Maybe the problem with the Blockade Runner was that the Rebels just weren't prepared enough. Captain Antilles (or whoever it was, again) was stuck with the troops he had on board, not prepared enough for the forces that boarded him (or possibly prepared for a different encounter, perhaps one he would have had later, perhaps one that happened earlier and which they had survived). With not one flicker of recognition (and a big guy in black with an "auto-erotic-asphyxiation-by-proxy" fetish helping the wannabee-clones to concentrate on the task at hand), the classical Rebel defence method was doomed...
Yes, yes, that explains it all quite nicely. I don't know why I didn't think of it before...
[1] Incidentally, the former better than the latter... I mean if you were in control can you really trust the Troopers, after what you did with their Clone predecessors to the Jedi? Make sure you can cut them down, thus unruly lot of of non-clone recruits. With the only practical physical requirement being that they fit the armour, and ideally not be noticeably short.