We’re missing a few ghosts.
Reasoning: their bodies are completely eliminated in one manner or another. (Disintegration, burnination, glitching out of existence via angsty Heart powers.)
Their physical bodies are what the dream bubbles use. The various doomed timelines are physically moved to the dream bubbles. If there is no body to use, then you’re either left with the standard definition of a ghost, or they simply cease to exist. The latter option seems more likely.
After all, when Lord English destroys the dream bubbles, he kills the ghosts to whom that bubble belongs. Without that containment, the souls/whatever of the dead simply cease to exist. They need an anchor.
In essence, anyone without a body experiences immediate second death because the dream bubbles have nothing to anchor the person to.
The dead dave shown in the flash was thrown into lava, but still went into the dream bubble while he was still dead in his room. I think the rule is that someone must be dead before the body disappears, which was not the case for the above three.
So what this would mean is that the dream bubble is created at the exact moment of death. For the Game Over versions of Kanaya, Karkat, and Dirk, their deaths happened because their bodies were destroyed, thus eliminating that anchor.