A comprehension gap is the length of time between a correct assessment and incorrect knowledge or blatantly wrong belief(s).
For example, a person believes there is nothing wrong when it comes to abortion. Later in life, the person starts to feel partial birth abortion is wrong. Over the years, this person will become involved with religion. He is told abortion is wrong and he starts to believe it. Then he has a full conversion when he feels abortion is the evil murder of innocent children in the womb. The duration between 'nothing wrong when it comes to abortion' and the correct assessment 'abortion is evil murder', is called the comprehension gap.
Liberals have been shown to have a higher than average comprehension gap, this is evident in many of their ideas, like the legalization of drugs. After the effect of these policies is demonstrated liberals will often abandon their position.