If it's already done and there are no forseeable complications (as in healing issues), don't worry about it. Your dog will most likely not go through hormonal changes or grow the same way others will, but there really isn't anything you can do about it.
Here's something that shows risks as well as benefits for you:
When To Spay When To Neuter
The reason that the Humane Society probably did it so early is so they can adopt out, not actively follow up to make sure the spay/neuter actually happened, and not worry about more puppies coming from this pup.