Originally, the IIHS offset
Originally, the IIHS offset frontal impact test was to simulate all of the common front-end crashes that didn't involve a head-on (grille-to-grille) collision. What they've learned is that while that test went a long way towards improving safety, those deaths still involved in this type of collision were not being addressed. Given that IIHS is a private institution paid for by insurance companies interested in keeping costs down by raising the safety bar in vehicles, it makes sense that they'd continually evolve their tests. Unlike the fed-gov NHTSA tests, which rarely change at all and give nearly every car a perfect score every time.
The fact is, auto makers adapt to meet the standards set by the crash tests the various institutions perform, so improving the tests improves safety provided the older tests are not disregarded.