Oxbow, its a great question, and a really simple answer, any grey market machine was originally built for some other countries market except where its at currently. Lets take hitachi for example, they build and ship machines worldwide, same as everyone else, like cat, case, deere, and everyone making machines, most have plants all over the world and the build machines for every countries markets. My model of hitachi was never sold in north america, it was built for say, europe or germany, japan or some place like that, not canada or the USA, so for mine, they don't have parts for that exact machine here in this country. But if you went to say japan, dealers there would have everything for them sitting on the shelves. Now some machines have similar parts in them here, but not all the exact parts, somethings were different, take my hitachi for example, its a 100 and has what they call a large single hydraulic pump in it, the almost similar machine built for the our market was a 120, that machine has a twin pump in, to give it added lift and boost over the 100 and the pumps are not inter changeable at all, too many things are different with the valving and electrical.
Now how did the machine get here in the first place, some jockey bought it used and imported a used machine and sold it here either privately or on auction, equipment ships worldwide anyhow, depending on who's markets are hot and where someone can make a buck off the stuff, so whats the big deal. Well some dealers didn't get their cut off the sale of the new machine originally and since some parts are different, they don't want to mess with stocking or getting the parts to fix them, but rather your supposed to trade it off for a unit they sold new or have parts access for in their network of dealerships, is about the nuts and bolts of it.
Now your next question is what's different between a machine for north american markets and japan's, korean, australia, germany or any place else, its complicated, it could be emissions, regulations, common stocking parts and manufacturers in those countries, it could be legality issues, or dozens of other reasons, or lets say, north american dealers decided they didn't need a machine or want to sell two models of machines so close in size so they just ordered one and not the other, who knows really unless your involved with management at the time.
I hope I didn't confuse you more than you were before reading this.