Heat of Combustion examples
You have to consider the $3$D structure and sterics when dealing with cyclic compounds.  If you look at the geometry of cyclohexene below you can that the ring is not symmetrical, thus the placement of the vinical methyl groups in regard to the double bond matters.  

In (X) when the vinical methyl groups are both on doubly bonded carbons they are forced into a coplanar arrangement which is unfavorable and maximally strains the ring's double bond and the methyl group bonds.  In (Y) when they are opposite the double bond the methyl groups are optimally spaced apart and steric strain on the ring and methyl bonds is at a minimum.  When they are not located on the double bond they are neither at a maximum or minimum, just in between so (Z) just falls in the middle.  There are of course other conformers of cyclohexene but the relation will still hold.

The question could have also thrown in a fourth isomer where one methyl group was on a double bonded carbon, but it would not be easy to rank it above or below (Z) without computational methods.