I'm not sure of the code that calculates the total number of pets wanted (something I'll have to ask Weena and Breanni about) but my best guess is that the Treasure Goblin (not yet implemented) isn't being considered as part of the total. Although it's listed under "wanted" and counts as one of the 13, it's not obtainable yet so it's not added into your total "601 pets wanted". Hope that made sense!
Feel free to browse through Perks N Peeves
, my vanity pet collecting blog!