My assumption is the pistol load is a little hotter than the rifle load to better dance on the edge of subsonic?
I would phrase it a little differently. They engineered the load to deliver the most energy out of a pistol, while staying reliably subsonic.
When I talked to the product manager, he believed they used a six inch barrel as the test barrel. So, a shorter barrel would deliver somewhat lower velocities. From my experience, a four inch barrel would deliver about 90 fps lower velocity.
That said, I think you are precisely correct. The pistol load is loaded to a higher energy to get the most out of a pistol barrel, without going supersonic.