Ahh... I would say most likely the method of vaporization... if I understand the volcano well it reuses the same air over and over and hot air actually does the vaporizing.
I'm not sure about yours, but if it's only a hot plate heating then of course since you suck on it at the rate it vaporises, it dosen't have time to saturate the air and condense.
Like I said THC vapors are strange somehow... butane oil on a charcoal does the same effect, it goes up invisible, then as the mushroom heat cloud evolves it gets white, but as it disperses it seems to vanish in the air... one would expect that it would condense further and fall, but no it just disapears evenly in the air and coats the room evenly. Hot vapors also can remain in an ice cup for quite a long time, the vapor becomes very cold, yet it's still airborne.
I'm not sure I have a scientific explanation for this, but there must be one ; you can even "drink" the vapors from the ice cup and it looks like CO2 vapors from dry ice.