General - I'm not sure. I know the owner operators of the Armoury that made the stuff, and sometimes their business practices baffle me. I would think they would have been able to sell more of the suits if they were screen accurate. The only reason I can think of that they aren't, is maybe they're prevented from doing so as a part of their contract...but who knows.
JP05 - I'm very jealous. I want one of the helmets so bad, but I'm not willing to accept the inaccuracies. I'm guessing your helmet is one of the generic Death Dealer helmets, and not one of the Elder ones.
But you're right, the replica and screen used helmets are not the same, and the armour is different as well.
Screen used helmet:
Replica helmet sold by the armoury:
The shape of the front "wings" is different, the embossed celtic knot is replaced with a flame pattern, and the ear medallions are missing.
Screen used armour:
Replica armour:
The embossed celtic knotwork has all be replaced by the flame pattern. The Embossing on the guantlets is different. The shape of gorget is slightly different - no biggie, but the interior collar lames are missing. The movie breastplate didn't have have that seam at the midpoint, and the Death Dealers in the movie didn't have greaves or sabotons...or chainmail