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My own experience with GPS units has been limited to older models with no computer connection -- or maybe there was over serial... certainly my Grandad has never had a computer to try it! That sounds like a really odd discrepancy, perhaps try importing the route into different software and see how it varies, does Garmin offer a desktop or web app that you could trail? My concern is that is actually pretty dangerous if it is reporting differently in the field, though I suppose the actual co-ordinates should be accurate... but then how would you know?