Well, of course different speaker designs, as you said. Also the reflectivity of the rear wall and what is in between them, and how optimized they are. I see photos of your system, but those do not look like Legacy Focus XD speakers. So I am a bit confused.
I also think a lot of systems are probably not well optimized. It takes not only patience but good listening skills. It can take years to easily perceive holes in a soundstage.
From your description it sounds like the imaging in your system is not being constrained by the rear wall, because you have damped them. Often an undampened rear wall can truncate the sound stages .
Interference with a lot of equipment also can break up the image… I say this because of your photo. I am not sure if you had tried it, but I would try moving the equipment to the side, and dampening all the was to the floor… and the back lower corner. To avoid a huge investment to find out… I would get some cheap long interconnects to go between the preamp and amp. If it has a big improvement then invest in really good interconnects.
Recently I went over to a friends house who was working to optimize his system. I could easily hear the equipment hole… we removed the equipment and voile gone. I added a big stack of couch cushions on the back wall and the imaging improved a lot and back into the wall. I could hear the problems in seconds… and to his delight he could hear the better results immediately.
So, I’m guessing, different speaker designs, suboptimal setups are the most common.