Unstable body structure doesn't really mean a bad star rating. Rating is capped on the basis of injuries faced by dummies. If crash dummy gets acute impact on head or chest, then no matter how stable body shell is, car will get a poor rating. Also, body shell stability is analysed after crash, it just means body shell is no more able to withstand further loading. So if at ODB 64, a car shell is rated unstable, it is more likely to fail a crash test above 64 kmph. Actually crash often has relative speeds of two vehicles. Which are usually higher than the crash test speeds. Also, sometimes objects involved in crash are non deformable(trees, pillers, even some trucks and buses). So even if the car has a 5 star rating and stable body shell, accidents can still kill people inside the car.