We need to focus here on the fact that there are 2 "set of steps". Every set contains 3+6+9+12 =30 blocks and hence two such "sets" would have 60 blocks.
The bias in your observation is the fact that you have assumed 4x4x3 to be a packed cuboid. However the set of steps is a part of it and not the representation of the full cuboid.
In fact that's why the total block count is 30 and not 48.