That does sound odd. If it's on hot and cold, I think you can
discard the thermal expansion theory. Here are two guesses. One
would be that he routinely did this, figuring it's easier to get the
tee on the main in the neighborhood, and then fit up to and through
the floor. Some plumbers back then carried nipple packs that went up
The second guess takes me back to installing threaded black pipe
for gas. When roughing in, I'd take as many measurements back to the
shop so I could use the big threading machine. It's so easy to be off
using this method, that I'd just be happy to hit the right joist
space, saving the critical measurements for the job site. While I
might use a few more elbows, the time saved by pre-cutting at the shop
more than made up for it.
At any rate, threaded pipe isn't nearly as forgiving as copper, and
it took a little more skill to put it in.