Sadly I can’t do a good job of explaining it, I’d figured it out racing on the club 10s before modelling TTs was a thing.
But I googled “why is it faster to ride hard into a headwind” and found this article https://www.bikeradar.com/features/why-its-faster-to-ride-hard-into-a-headwind-than-with-a-tailwind/
Don’t worry about aero drag losses, because that isn’t the point of a TT, you need to consider time to cover distance.