The question is how repeatable is the NHTSA/TC testing and do you put all of your trust into just these specific test results to make a choice? For example, a lot of people buy seats strictly on the Crash Protection rating score in a Consumer Reports review, also.
Other factors include ease-of-installation, ease-of-use and features like a rear-facing tether. I always use a tether when forward facing, anyway. I know the seat is installed and used correctly. While impressive, untethered results are not really interesting to me, at least for my kids. Obviously they would be more interesting in other situations.
One very nice thing about Britax is that they continue to make innovations. If this is a genuine opportunity for improvement, I'm sure it will be addressed.