Answer:
In saturated fatty acid oxidation, the first step is a dehydrogenase reaction. This yields a trans double bond on carbons 2 and 3 from the CoA end. A product of the reaction is FADH2 which can be used to make ATP.
In oxidation of an unsaturated fatty acid, the double bond is not recognized by the dehydrogenase reaction therefore you have to use a secondary isomeration reaction to produce the same product as you did in saturated fatty acid oxidation. This skips the effective "first step" and does not produce FADH2 meaning less ATP is produce.