Not all alkenes were created equal. Like carbocations, alkenes are stabilized through a phenomenon called hyperconjugation.
Hyperconjugation allows adjacent -R groups (mostly C-C and C-H σ-bonds) to create shared molecular orbitals with π-bonds, stabilizing the bond.
So basically, the more –R groups attached to the double bond, the more stable the double bond will be.
Specifically, when it comes to di-substituted bonds, the order os stability is gem > trans > cis.
Example: Rank the following alkenes in order of lowest to highest heat of combustion.3m