Differences in connectivity between regions of the brain may explain why, Prof Zeman says.
Asked to picture an apple, for example, most people go through a succession of steps, including nudging the brain to remember what an apple looks like and activating the brain to create an image of it.
But in those with aphantasia, that process can break down at any point.
"Thoughts remain thoughts," Prof Zeman says, "whereas for others, thought translates into sensory terms."
While aphantasics think about memories, other people are able to recollect and live those memories.
But intriguingly, many aphantasics can visualise images while dreaming - probably because it is a more spontaneous task beginning deep down in the brain, Prof Zeman says.
And aphantasia can have benefits. It can have a protective effect on someone's mental health, because they are more likely to live in the moment and less likely to imagine frightening or stressful events, for example.
But for Prof Zeman, "the big surprise" was aphantasic artists, who told him their struggle to visualise imagery gave them an extra incentive to make art, by using the canvas as their mind's eye.
Normally, it's hyperphantasics who are more likely to be creative, like Geraldine van Heemstra.