Yeah, that's right people reacts different to mind altering molecules because their personality is different, and my hunch is that is also because of genetic factors and the way their brain is constituted, it is true you can hallucinate from high doses of cannabis, but the term hallucination is wrong, first cannabis has several psychoactive compounds, not just THC and the concentrations of these vary from plant to plant.
Also cannabis could have been laced with something else thus producing other very different set of effects.
And now returning to the hallucination term, I don't remember the exact meaning, but not even hallucinogenics make you hallucinate, they might make you see strange things, like melting or breathing walls, or produce some sensory distortion so 2 senses get mixed (synesthesia) spelling? it can also produce profound mood and consciousness states, but all the time you know that it is a drug induced state you don't believe your hallucinations so I suppose that's why they shouldn't be called hallucinogens.
Other alkaloids however produce full blown hallucinations like the ones people believe the popular drugs produce, and the worse thing is most of these things are legal and very easy to get.
Well so now that we have an understanding of what hallucinogens do, cannabis in high doses can produce close eyed visuals, geometric patterns, kaleidoscope like visions, and also for example if you played doom before, these visuals can get doom like characteristics (pixelated walls, etc)
Cannabis can also produce very strange patterns of thought, you can either become really concentrated to the point of obsession with a topic, or completely distracted about everything, your thoughts could race to your mind and accumulate because you can't process them that fast, if you really smoke a heroic dose with your eyes open you can see things like for example a shadow that looks like a volkswagen, a couple of trees that look like a forest, I think this is because your imagination and capacity to dream is heightened by the drug induced state, but you won't see things that aren't there or won't see things that are there :)
to end this very long reply I'd say to conclude that maybe very few people has a different set of effects with cannabis than those that are usual in most of people I suppose it'd be interesting to measure their brainwaves or maybe to use more sofisticated ways of knowing how their brains are working like x ray scanning, rmn or pet, It'd indeed be very useful to understand further how our mind and brian works