I'm going to chime in here, because I realized this as part of my research. If you look at the IIHS testing site, you can see a detailed breakdown of how those scores were determined.
2018 Subaru Crosstrek
2018 Subaru Impreza
You can click on the headlights breakdown at the bottom to show you what the test results were. I'm going to assume we'll talk about the highest trim of the 2018, which is the 2.0 Limited w/eyesight built after October 2017.
Basically, from what I can tell at least from the IIHS test is that for some reason, the Crosstrek Headlights have more less 'throw' than those in the Impreza. What's ironic is that the test for the Impreza states that it has some glare, where I do not see it on the Crosstrek even though the lights on the crosstrek are higher. Maybe this has to do with the way the lights are angled.
Anyhow, that appears to be the reason, at least from the IIHS test, the Crosstrek's lights aren't projecting as far as they are expecting from their tests. That being said, I've driven the Crosstrek, and the lights are almost unnervingly bright (compared to an old '04 Civic).
The two cars are similar enough (exact same platform) that I would not really let this 'classification' bother me. It's like someone getting 86% as an A and 85% as a B. I don't actually think there is a significant difference between the impreza and the crosstrek given the headlight comparison.