I saw in this post from 2011 (http://www.lackeyccg.com/forum/index.php?topic=1184.msg8752#msg8752) that chance probabilities in pack definitions currently supported integers only, but that Trevor was changing it at the time.
I don't know how much good it does to mention it here, but probabilities with a decimal component break the pack loader and make it always select the first chance. I encountered this bug while improving and adding to the Magic plugin's pack definitions, and it's going to make things... tricky.