I'm using Adode Reader 6.0, and it doesn't look weird or anything, either.

The only thing that I see that it *might* be is the arrow that goes between a1 and b4 on the diagram is drawn over whited out areas on the arrows below (probably to prevent confusing black blobs where so many arrows cross over each other). What program/filetype are you using to view the spec? It might be rendering stuff weird...