I do notice it, I'm just wondering why those elements?
I haven't a clue. I just went off of his ideas and tried to fancy it up a little.
Though, here's my theory:

: our ability to measure things, eg. a line. (though a line isn't exactly thermodynamic)

: perception of shape.

: gravity needs volume and mass to create weight. volume is consistent of the three dimensions.

: self explanatory. supposedly the fourth dimension.

: all things ethereal. in this case, the invisible pink unicorn.
But alas, that's just a guess.