I mean, are people really "investing the difference," or is it more likely there is no difference, because they simply cannot afford the machine at their salary?
Some of both. People who are in a better financial place are doing the former, people in a place where they can't afford the whole cost up front but want or need it for some reason are doing the latter.
Dude, that sounds a lot like "mom and dad loaned me the money for my car and I am paying them back."
You seriously must live under a rock if you've never heard of a 0% APR offer for a vehicle loan. There were also entire websites about them linked earlier in this thread, multiple times. (And a lot of moms and dads would charge their kids interest, anyway.)