The difference between pure copper and pure aluminum is significant when you are talking 225 Amps of current, enough to generate excessive heat.
Pure copper has about 38% less resistance than the same size aluminum.
My guess is that they are using an alloy mix that has resistance closer to copper.
The only reason to use higher resistance wire than pure copper ( other than cost) is to form a current limiting action at the cost of excessive heat.
Aluminum house wiring in the old days was downright dangerous especially with high current stuff.
BTW, I have a piece of flaked off wire insulation from my AC-225 and will check at what temperature it starts to melt. Let me just say this, i held a lighter to one of those flaked off pieces and guess what. It not only melted but caught fire too. Those engineers are not formulating the wiring varnish very well!