I think I read elsewhere on this site that one of the reasons the satnav changed from Garmin to 'here' is its more compatible with the 'over speed' warnings now legally required.
It is difficult to know whether the car is displaying a speed limit based on sat nav gps data, or having 'read' a speed roundel. I think it normally works from gps data, but if it 'sees' a speed roundel for a lower speed it uses that instead . Which is logical and errs on the side of caution. It might be a genuine temporary speed limit for road works etc. I dont think it would overide the mapping /gps data for that stretch of road if it 'sees' a higher speed limit sign.
Unfortunately it doesnt use gps data for measuring the real speed of the car . Speed bongs are sounded based on the car speedometer, which is typically a few mph/kph slower. ( My standalone garmin displays accurate car speed. I do miss that.)
I have seen it in action in countries where villages etc may not display much in the way of speed limit roundels . Drivers are expected to know the 'urban' speed limit that begins at the village name board, and ends at the nameboard cancelation. . The roundel on the drivers display changes to the speed limits at pretty much the right place, even if no physical speed limit roundels are displayed .Occasionally it might drop the speed from say 50 kph to 30 kph in the village centre when I had seen no speed roundels. Maybe there was some subtle indication known only to locals, such as the presence of lamposts . But the satnav knew. And the 'annoying beeps' saved me , as intended.
Sometimes the gps data displays a conditional speed limit sign such as '30 kph in snow' which is definitely not physically present (at least in summer. I have been back and double checked
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Speed limit gps data is inputted and updated by human beings ,so subject to error. Some countries mapping may be better than others, and Garmin often get this inputting done in China, which will lack local knowledge. So I never rely entirely on satnav speed limits.
