There is no guarantee in that, which is why you will always see some stablecoins quoting a little less than the value of a USD or a little higher than the value of a USD. Most claim that they have their stablecoin backed by the USD, but this is truly not so, and it is due to many factors including their selfish gain in rate manipulation and what you just explained above.

I can only believe that a Stablecoin will always be equal to $1 if the stablecoin has its own blockchain and is truly backed by the USD.