Why does a larger amount of my balance get locked when sending TUBEs?

When sending TUBEs from one address to another, during the following 10 confirmations on the blockchain, the amount of this transaction will be 'locked' from your balance. It occasionally happens that the locked amount is higher than what you sent, however.

Since this gets asked a lot here's a super simplified explanation that should be relatable to all on why does the wallet locks X amount when I only want to send Y.

Imagine that you walk to a cash register in a supermarket, your groceries amount to Y coins (the amount to transfer) which you'll need to pay the store (the recipient), you only have one X coin bill (the inputs in your wallet) to give out (it's larger than what you owe so that X-Y=Z), you give your X coin bill to the cashier (the blockchain) that then deposits Y for the shop and gives you back Z as change. You can't spend the remaining Z before the cashier has handed it back to you, can you?

This is essentially the same reason why sometimes a small transaction seems to lock a huge sum (you only had a huge bill to give out).