Even though you won the overall match, the rating system also looks at how you won, not just whether you won.

If your opponent had a much lower rating (which they did), and you lost some ends, the system sees that as underperforming.

To actually gain points in that situation, you would’ve needed to win all or nearly all the ends, showing a clear skill difference.

So in short:
✅ You won the match = good
❌ But lost some ends to a low-rated opponent = rating drop

Hope that clears it up for you.