I think it's because the pool stuff is sold and installed by the pool people (with no need for a certified HVAC person), and the HVAC stuff is installed by a HV

I think it's because the pool stuff is sold and installed by the pool people (with no need for a certified HVAC person), and the HVAC stuff is installed by a HVAC person with no need for plumbing. It's like why you get certain procedures from the cardiologist in the cath lab, and some procedures by a cardiac surgeon in the OR, sometimes the same procedure.