Did you look at the…
Did you look at the methodology of testing from the different groups. IIHS does their testing in labs under controlled situations to "simulate real world events". Since the 1970s NHTSA studies real world crashes. Most of their research comes from real crashes with real drivers on real highways and country roads across the country. A Nissan Altima being pulled into a wall on a sled at exactly 35 mph is not going to provide the same data as a Nissan Altima being driven by a 25 year old on their phone crashing into a jersey barrier.
Also, having been in the insurance industry for over a decade and attending those showcase events saying IIHS is 100% funded by the insurance industry is misleading. NHTSA doesn't get handed the keys to BMWs, Cadillacs, and Lexus as a courtesy for spending an afternoon driving one around the track. At NHTSA I wasn't given a $200 bottle of scotch at an underwriters conference.