Sometimes this can happen with any unit, because of up to two reasons. (i) Maps are notoriously inaccurate as to the actual positions of the roads at times. (ii) Non-profession GPS units usually have satellite accuracy of up to 15 metres. Let's assume you have accuracy of 10 metres (meaning you could be one way or another) and the road on the map is several metres off its actual position. If the errors are cumulative in the same direction, the unit's 'snap to road' feature may well wrongly place you on a parallel service road instead of the highway or vice versa even if the roads are 15-20 metres apart. In the case of elevated roads, laterally the two roads may only be several metres apart and although you have entered the highway the unit assumes you are still on the service road for instance. Only when the two roads diverge sufficiently will the snap to road feature move you to the other more likely road.