Why sometimes my car slows down going downhill?

I drive a small automatic car. For the most part, it struggles a bit with steep uphills, and picks up speed when going downhill. That's to be expected.

But then I noticed some downhills actually slow it down. I've noticed during a two-hour motorway trip I often do to visit family, there are some specific downhills, not all, just some, where my car always slows down and I have no idea why.

Ideas?