Unconfirmed Rewards = Rewards you've earned from blocks without full confirmation.  Depending on how long it's been since the pool last had a block, how many blocks have been found on the network since, and how long the pool leaves them in "unconfirmed", this number can in fact be 0.  Just based off what "Unconfirmed Rewards" have meant for every other pool I've seen it on over the last 3 years.