That would make a lot of sense. Civil defence must be being pessimistic in there predictions, as you should be. It's better to be away from a coastline if there might be a tsunami and there isn't, then the other way way around.

Then why is it still reported that waves 3-5 meters are expected?

Civil Defence are doing their job, getting people away from the coastline. There maybe Tsunami's there may not be, better not to take any risks. Must be protocol when any of the Tsunami alarms are triggered.

Also because your coastline can fundamentally change how it affects you - the gauges near Christchurch measured 0.5m appx at the highest, whereas an (empty, thank goodness) house at the end of a funnel shaped bay in the same vicinity got hit by an estimated 2 - 3m wave.