Shapeways' Steels

Right now, Shapeways has 8 kinds of steel:

GROUP 1
Stainless
Matte Bronze, Polished Bronze

GROUP 2
Matte Black, Polished Grey
Matte Gold, Polished Gold
Polished Nickel

They're in 2 groups based on the cost. But what's the difference between the steels? Their website isn't very helpful. I think the second group is the plated group. From plain steel, they can plate it black, gold or nickel, then polish it or not.

Then what's the difference between plain stainless and the bronzes? I can't figure this out since plain stainless is already 40% bronze. Are they plating it with more bronze? I don't think so, since it would cost more and put it in the second group. So, I tried ordering something in polished bronze:

The left is plain stainless steel, and the right is polished bronze. Their stainless is already a bit yellow, but the bronze is dark brown. Polished bronze looks a bit rougher than stainless. Hmm so there's a difference, but I still don't know how bronze is made.

More on their metals: it looks like all their other metals (namely, platinum, gold, silver, precious plated metal, brass and bronze, excluding experimental aluminium) are made by the same process. They print in wax, make a plaster mould around it, then cast that metal. Only stainless steel is printed directly in the metal (although it's very porous). You can also buy the wax form and cast it yourself. These metals are more labour-intensive and hence more expensive, but much more detailed than stainless steel.