I've done all that Garmin support asked but still the same

quite often the satnav suggests a silly route. It's not new roads it's not traffic.

eg yesterday on M27 coming from Southampton to Lee on the Solent it said to exit at Jn 11, but knowing better we exited at Jn 9, and lo and behold once the daft thing recalculated the route it suddenly declares our route is 10mins quicker ! Which of course it was. As anyone who looked at a map for a moment could see.

What's going on here, I just don't understand how a satnav can fail to get this right?