I thought pak128 was called a half-height pak set, since one height level is only half the clearance needed for vehicles. To levels are needed to get the full height required for vehicles to pass under. Pak64 is a double-height pak set, because the height difference is either the full height needed for vehicles to pass, or double that. By contrast, a legacy single height pak set is one where the difference in height between two tiles can only be one, which must therefore also be considered enough of a difference to let vehicles pass under. Or to turn it the other way around, which is how I think the terms came about: when the feature was introduced that allowed slopes of two different steepnesses, half-height pak sets used it to create height differences half the height of the original, while double-height pak sets introduced height differences twice as big as the original.