Question

explain why it is formed that every cistron is a gene but every gene is not a cistron

Answer 1

A cistron is an alternative term to a gene. The word cistron is used to emphasize that genes exhibit a specific behavior in a cis-trans test; distinct positions (or loci) within a genome are cistronic.

Answer 2

A cistron nothing but an alternative term to a gene. It is a section of DNA or RNA which codes a specific polypeptide in a protein chain. Cistron is the largest element in a gene which  codes a polypeptide during protein synthesis. Cistron refers to a test called cis-trans test, which is similar to a complementation test. “Cistron” is a unit of function.