I had a similar experience but had an opposite takeaway. I learned that my car, in dry, daylight conditions, could comfortably go 20±5 mph faster around curves than those signs suggest.
Those signs aren’t just for whether you can go around a curve comfortably. It is if you can see and/or stop safely if something happens. If there is a broken-down car in the lane, you will see it way off at 65 mph, but if you are going around a blind curve comfortably at 65 you might plow into it or skid off the road.
Those signs can also be posted as advice for adverse conditions like snow and rain. (Of course, ice = all bets off)