Maybe it was on his site that I read it, but I'm almost positive that they made the hard levels first, and they added the easier versions to make the learning curve smoother. It makes a lot of sense, since the easy levels are the same but with (generally) 20 of each skill, while the harder versions clearly had thought put into them.
It makes more sense that way then making a bunch of layouts, giving 20 of each skill, then thinking "Oh, what's the least obvious possible route that uses not too many skills?" I could see this working, experimenting with all skills available and finding the simplest but non-obvious solution (I'm sure a good number of levels are designed this way), but not for all levels. Some were clearly designed for the harder version, like "It's hero time!", "Compression Method 1", and "Down, along, up. In that order" to name a few. Maybe it was a mix of the two methods?