I saw an ad for a car that said "US Govt 5 star rating front and side impact".But the side rating was marginal?

Here are the details: I went to the insurance institute website and looked up the car. The front impact rating was "good" (as you know, the grades avail. are good/average/marginal/poor). OTOH, the side rating was only marginal (next to the lowest rating and worse than "average."). What gives? How can a car get a 5 star rating on front & side impact and do relatively poorly on the side impact test? Sorry, but marginal doesn't sound like a five star performance to me.