I think it's just fitness, plain and simple.
I rode some decent mileage during the 2017-18 winter, but it was all outdoors and very slow through snow and ice. I also forgot that I was no longer riding summer mileage, ate a bunch of food, lost muscle, and gained a little bit of weight. I worked hard to gain fitness back, and it took most of the summer to do so.
This last winter (which was absolutely awful), I bought a trainer, did some mildly structured sessions of 40-60 minutes, ~3 times a week, and tried to eat normal people amounts of food rather than athlete quantities. With only 900 outdoor miles this year, I'm already feeling much better, my average speeds are increasing, and I'm hitting some PRs.
For years, I resisted the trainer. I hate the trainer, but it's been worth the benefits for transitioning from slow outdoor winter riding into fast summer road cycling.