This statement could be understood to mean a couple things. Just to make sure it is clear for people, here's the explanation that I've used in the past:
The "staked" amount that is listed on the overview screen in the client (and in the output of the
getinfo RPC command) doesn't represent how many shares are available to use to solve a block, but instead are shares that have already been used to solve a block. The total number of shares used in the inputs that solved the block show up in the "staked" count for 5020 blocks, at which point the reward is considered "mature" and it is deposited (along with the inputs previously used to solve the block) back in the "balance' amount.