Calculus is incredibly complicated. Keep in mind it wasn't properly formalised until the late 19th century. It depends a lot on the book and the student, but es

Calculus is incredibly complicated. Keep in mind it wasn't properly formalised until the late 19th century. It depends a lot on the book and the student, but essentially, to read Calculus I+2\varepsilon you need a solid foundation in Calculus I+\varepsilon. So, usually most of the skipping is because the text assumes the prior is known and digested (or it is just a bad book.)