In JIT1, a consumer will constantly demand whatever good it consumes, and every piece of good the producer creates will go to the next station, often causing an endless stream of "station overcrowded"-messages. That is until the consumers storage is filled up. In that case, the demand will switch off and no new goods need to be transportet. Everything that's still on route would still go, filling the consumers storage even more. After a while, the consumer will have consumed enough so the storage is not filled anymore. Demand will switch on, and transport goes at full rate once again. So if you build too many vehicles, they might all transport goods, but at some point, there won't be anything to do for them, and they will just sit around doing nothing.
In JIT2, the consumer will try to demand as many goods as they actually need. So the amount of goods you can transport is not only capped by how much a producer produces, but by how much the consumer needs as well - whichever is less. The benefit is that the consumer won't fill up completely, so while you might only be able to transport a bit less, there won't be any weird pauses in transportation.
In a sense, JIT2 does what your bonus question is about, you just need to read the numbers differently. As in, the production per month is what COULD be produced by that producer, but since the demand is less then that, it doesn't. Only works in one direction though, since production per month is not directly increased with higher demand. (Indirectly is another - complicated - story, and I'm not sure I qualify for explaining that)
