A P30+ with 19 captures??? I've got no idea how that happens. How can you drop that much money on a MFDB and then put less than a roll of film's worth through it?!?
Several ways, only one of which is the "collector" who doesn't use their system before upgrading it.
If someone buys a new Team Phase One back and it's delivered with some defect, even if it is quite minor, it is replaced rather than repaired. The unit with the defect then is shipped back to the factory for repair. It could be something as small as a button whose clicking action wasn't quite firm enough. This is pretty rare but if/when it does that unit can not any longer be sold as new – even if it only has e.g. 19 captures on it.
Another situation is purchases during a product launch. Several Team Phase One products were announced before they were ready to ship. In such cases there was usually a provision to buy the announced product and receive a loaner back in the meantime. For instance if you bought an IQ160 when they were announced you got a P65+ for several weeks while waiting for the IQ160 to ship. Some clients do not end up using their loaner much, either because the final product shipped faster than expected, or because they already owned a back and were just using the loaner as a backup, or whatever reason. Those backs again, still could not be sold as "new" again and end up in the refurb stock.
There are a few other ways this can happen too.
Personally I don't think shot count is all that important. I'd consider warranty, condition, history of use, and support much more important. Shot count can serve as a pretty good proxy/indication of history of use, but it's not perfect. For instance I'd rather have a back with 100,000 shots that came from a product photography studio where it never came off the camera stand vs a hiker who was constantly roughing the back around (these backs can take heat
and a beating
, but the studio back is obviously going to be closer to new in overall condition but push-comes-to-shove I'd still rather own a back that lived a boring life).