More to the point... The 6 ft tsunami height is the height of the wave AT SEA. As the wave approaches shore and gets into shallower water it will build up and break several times higher than that.
Another difference is the period. Instead of a normal wave with a period of only a few seconds... A tsunami could have a period of several minutes. That is, the wave comes in, and just KEEPS coming...
so a 6 ft tsunami could be, on shore, 12-18 ft?