GPS measures linear velocity between point A to B whereas in real time youre not really travelling linear therefore a slight discrepancy. Also weather (overcast) affects the readings too.
Another school of thought says the meter over reads your speed and the GPS ones are accurate.
Generally they dont stop you at 130kmph as far as my experience goes.
I drive at 110kmph that is the one at which im most comfortable and can relax.
High speed makes one tired which kills the purpose of travelling.