USDT purchases and associated gains indicated

Can anyone in the community answer this? Why is that more often than not, when I must purchase USDT with fiat to buy a token in a USDT pair, that the transaction shows up in CT a gain on the USDT side of the transaction. Sometimes it shows as a nominal fee of a few dollars and sometimes it is significant (my transaction sizes are not huge - usually no more than 1,000.00 USD). Regardless of the fee amount, if it is USD that I’m using to buy USDT, and USDT is tied to the value of the dollar, why would there be any appreciable gain shown???