"Every (sinful) deed can be expiated; and the fast is for Me, so I will give the reward for it."

 [Bukhari]

There is a privilege in Siyaam; people's rights will be recompensed from man's acts of worship on the Day of Resurrection. For example, if you backbit someone, the rewards of some of your prayers will be added to the account of the person you backbit.
A man once accused someone of backbiting him, so the latter said, "If I were to backbite someone I would backbite my own father and mother, because they are more entitled to my rewards, but not you.
Siyaam is the only act of worship whose reward will not be used to recompense people's rights on the Day of Resurrection according to the aforementioned Hadith. The reward of your Siyaam is protected from being used in recompensing for your sins.