This is like the old story of the guy with two watches who could never be sure of the time.
Bike computers, both wheel based and GPS based, have a certain programmed lag before the stop the clock when you stop. just about all, give you a minute or so to allow for stops signs, red lights or just quick breathers, while considering you to be in continuous motion.
This can affect the breakdown of riding time and stopped time, and indirectly introduce "errors" in average speed. There's no standard in what defines a pause vs. a stop, so a difference of opinion between Sigma and your GPS unit can account for what you're seeing.
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