If this is true, then why do we even have a limit setting by the way? Sorry to diverge from topic:)
That's how ethereum network works.
Note that if you set the gas limit to higher than 21,000, you won't lose anything.
For example if you set the gas limit to 30,000 and the gas price to 100 Gwei, 0.003 ETH is deducted from your balance as transaction fee. Once the transaction is confirmed 0.0009 ETH will return to your wallet.
If there's a token in the transaction, the required gas would be higher than 21,000.
Since the extra gas will return to your wallet, it's recommended to set the gas limit to higher amounts to not lose money.
Let's say a token transaction needs 80,000 unit of gas.
If I set the gas limit to 50,000, the transaction won't be confirmed and I will lose the 50,000 unit of gas.
If I set the gas limit to 100,000, 80,000 unit of gas is consumed and the extra 20,000 unit of gas will return to my wallet.