What could be the reason for knowledge gap in people

The kind of food and activities eaten and done in childhood can affect the health of the brain in future.
How intelligent a person is depends on how the brain was trained and how much interest a person has in something.
Every healthy baby is born the same way, but the activities they do along the way, the foundation of their education, their interest and everything they do in their day-to-day life help shape their brain. This is why when they are grown, one kid can read a book once and understand and remember what he read, while another kid will have to read it multiple times.

Then, for the "interest" part, a person who finds chemicals interesting will find it easier to understand topics about chemicals or chemistry as a whole. The same thing with a person who finds coding interesting. It is thus interest and fascination that push them further into learning these fields.
This is why you can see a person make music so beautifully; he can make music out of any sound, and it will be beautiful, but he won't be able to know 3x50 without thinking hard.