Chunking of sensory input happens at a lower layer in the brain than consciousness. So if you have learned that two stimuli are the same then they might be indistinguishable to you unless you spend thousands of hours deliberately practicing distinguishing them even if there is a detectable difference, and even if you can distinguish stimuli that are just a bit further apart.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.