Stable coins are not same as the fiat currencies at all. USDT is an example, due to several reasons, the price of each usdt can be increased or decreased. For example, 2-3 years ago price was fallen to 0.8usd for each usdt. As far as I know, there is no chance for find an usd stable-coin which is guaranteed to to be always 1:1. The stability of the stable coins usually depends on the supply and demand and the amount of liquidity the team provided to back the price.