Sometimes when you make transactions your client will automatically create new addresses and split your funds between them. Why it does this I have no idea, maybe someone else can explain this.
This is why I recommend manually sending from a particular address, as one of your addresses may have a larger amount of funds held in it which you don't want disturbing, because from what I understand, stake will be reset to zero days if your wallet moves funds from it.
This is because an 'Input' amount of coins in the (or any) blockchain cannot be partially spent. Each incoming transaction in your wallet addresses is treated by the blockchain as a separate unit. You can send one (or more) out in total, and receive 'change ' if needed, in a separate address. If you wish to spend coins exactly matching some combination of your original inputs, the 'change' address is not required.