Seems to be influenced by the pieces on the board. Bill talks about it a little in the article. You do seem to get more | shaped pieces when you leave those spaces open on the board.
I don't think this is correct, nor that it can be evinced from the article. What it does say is that the sequences that led them to achieve their target show a higher incidence of I shapes. This is because all the ones that show less I shapes have been "pruned away" by the cost function, which favors I shapes.
This has some relationship with the anthropic principle: isn't it strange that, of all the possible universes, we ended up in the one that seems fine-tuned exactly for life as we know it?