You want the plastic response in the extruder head to be hard or crisp. Probably not the best words to describe the characteristic. What I am thinking though is kind of like getting air in the hydraulics of car brakes. They get spongy. For instance, if your hot end design was prone to trap a bubble of air, you would have to over step to get it going, and once you stop, the pressure from the bubble would keep pushing out plastic until the pressure equalizes. I have seen my hot end continue to spew out plastic after stopping stepping.
Also, if it's easy for air to get in the inlet where the filiment feeds in, melted plastic could continue to pour out of the chamber. Kind of like filling a straw and putting your thumb over the top. As long as you have a seal, the water won't flow out. As soon as you break the seal, the water pours out.
I am kind of shooting in the dark, but feel there are definitely some differences. My grandson is off at college with the hadron, so I can't really test it. I am currently traveling but perhaps when I get home in November, I can set up some definitive tests. I have some spare j heads.
Granted, if we can tweak the software to accommodate our individual printers characteristics, that's going to get us printing. Sometimes I am plagued with wanting to understand the root cause of issues.