How is this even surprising to anyone? Unless they are clones then every creature is slightly different and you would expect their vocalization to be unique as well, for example how often do you meet people who have the exact same voice? Only issue is our hearing probably isn't sharp enough, or optimized to the same frequencies/ranges, to pick up the differences in the sounds generated by most animals.
It's not surprising, but still fascinating. This property isn't limited to humans and wolves and dolphins, either. Scientists at Cornell showed that parrotlets have unique "names" given by their parents.
<a href="http://news.cornell.edu/stories/2011/07/parrots-learn-their-names-their-parents" rel="nofollow">http://news.cornell.edu/stories/2011/07/parrots-learn-their-...</a>