I don't know what was in their mind, but my guess is that it's due to automotive HVAC systems using flexible hoses rather than hard piping like stationary HVAC. The consequence of this is that those hoses aren't /truly/ impermeable, and it's commonplace for a vehicle HVAC system to need to be recharged after about 10 years of service life even with no "leaks". So, pretty much all gas going into the system will eventually end up in the atmosphere over time.