Scientists are still figuring that part out! They have found statistically significant differences in how people experience illusions like the above one. They think it might say something interesting about perception and variance in perception across individuals.

People working in AI are highly interested for the same reason (understanding how humans perceive the world might help them build AI that can too).

Merav Ahissar is a big name on the cog sci side of this research if anyone wants to dig into the (rather dense) science.