It is the difference between buy and sell price - the difference is pocketed by the crypto exchange.
Although currently the exchange Tether to fiat dollar is close to 1:1 the peg is a promise and a guarantee, as described in this article -
The danger in the stable coins (thought experiments and the real price of Bitcoin)
If a small group of people decide to cash out - sell bitcoin, get Tether, and attempt to convert to fiat, the 1:1 peg may be broken…