The human body has been proven to actually be capable of spot reduction, by controlling hormones. Hormones and genetics happen to be the reason why women have bigger hips and males have bigger stomaches, why some men have gynecomastia while some females have a more manly feature, thus it only makes sense to control these fat controlling hormones.

Okay, simple science aside...

The butt/glutes is sort of like a "toxic" marker in the body. The higher the level of toxic in the body, the higher chances of storing fat in these areas. Thus in order to reduce fat in the glutes, is to reduce the level of toxics in the body.

Also certain movements such as cycling, spinning or jogging actually cause you to store MORE fat around the gluteal areas, as a quicker energy source for the body. Just take a look at joggers. Notice how their bodies seem to be very, pear'ish? The increased stress from all the pounding from running, doesn't help either.

It's likely that you'll get better results from a combination of a bilateral and unilateral circuit, lower body weighted exercise with high repetitions and low rest time would work better. Also reduce toxicity levels with clean eating.