I’m going to take a guess that you do not have power meters on both the bikes and that this is the first time you’re seeing this kind of info. Weight is a critical ingredient in w/kg, but it doesn’t necessarily tell you why her w/kg is higher. Here’s my theory of what’s going on… all this assumes you’re riding the same course and duration. No doubt, w/kg will be lower on long rides:
You ride one at a time on the trainer, but in real life (IRL) you’re together. IRL you are motivated to ride to each other’s w/kg because you want to actually be riding together. In other words, IRL you’re not going to let her “kick your butt” (or she’s soft pedaling so that you keep up). On the trainer you’re riding to your own power choice, and you are just not putting out her power-to-weight…for any number of reasons, and probably more than one. She could just be more fit…or motivated. You could be not as inspired to put out a bigger effort.
Another factor could be drafting. Are you riding the same course and distance,…how does your actual time differ? Are you a better drafter and she’s pulling the whole time? That would impact it.
I think there are lots of variables here and unless you really make it scientific to equalize them then there is no one single reason. I’d be curious if you’re looking at Strava or Training Peaks for IRL. If so, look at your w/kg as well as TSS as an indirect reflection of how the effort you’re each putting out for the same ride. It also could inform you if on the same IRL ride you are working your tail off and she’s just coasting along. IRL motivation is a big deal for many. You’re probably not wanting her to drop you IRL, but in a simulation game, you might not find the need to hold power so compelling.
Hope this helps a bit.