Why the difference in the Legacy and Outback? In a word: headlights.
The IIHS structures their awards around several elements: crash tests, crash-avoidance systems, and headlights. Both Subarus aced their crash tests, earning Good results for the entire battery of tests (small and moderate front overlap on both sides, roof crush, side-impact, and head-restraints). Their standard EyeSight accident-avoidance automated emergency braking systems also performed well, successfully avoiding crashes at 12 and 25 mph, netting Superior ratings. (The pedestrian-detection function similarly earned a Superior rating, although pedestrian detection doesn't count toward the IIHS total score.)
Headlights have to earn a Good rating in order for a car to qualify for TSP+; headlights deemed merely Acceptable limit a car to a maximum TSP award. Headlights that are Marginal or lower knock a car out of TSP contention.
The Legacy's optional curve-adaptive headlights were deemed Good, while those same headlights in the Outback were only Acceptable, resulting in the different awards for the two models. For both cars, the base headlights also were Acceptable, meaning either model with the standard headlights would be a TSP (except that the IIHS grants a single model only its higher-level award).