The 'error' is not really error. This is very intentional. Auto makers in Germany can get in really big trouble if the speedometer is slow, so BMW makes theirs a little fast. Since your 330i has a higher power/weight ration than your 5ers, perhaps BMW used a larger margin -- again, to ensure compliance with traffic laws.
How realible is the speed reading from the GPS? I would look for something like a radar gun or a velocimeter from a physics lab instead.
I doubt you have anything about which to worry. Hopefully other E90 owners are able to confirm my hunch.
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