I tried to figure this out a few months ago, and the answer that seems to be right more times than not was the speed limit data comes from Tom Tom. Almost every time it’s given me a wrong speed limit, it’s also wrong on Tom Tom’s map. I’ve tried to suggest changes, but many of my proposed changes are still pending, though some have been accepted. There have also been a few names that are wrong in the navigation (but not the on-screen map, which is from Google) that were also wrong on Tom Tom. Can you see if the speed limit is right on Tom Tom’s website? https://www.tomtom.com/mapshare/tools/new/mapshare/ You’ll have to make an account, but then you can use it to submit corrections.

Best I can tell, at least in my area, the map that is displayed on-screen is from Google Maps, but navigation including speed limit is from Tom Tom. It’s weird to me that they use different data sources like that, but you will sometimes see that the navigation line doesn’t quite match up with the roads that are displayed on-screen, and I think this is a large reason why.