The working range between $USDC and USDT is narrowing as they consolidate their positions as top stable coins.
There was a time it was as volatile and disparate as $FDUSD.
The US, naturally has enforced policies to protect from competition stable coins issued by American companies, specially from foreign alternatives such as USDT.
Still, the exchange rate between USDC/USDT is not exactly 1:1, sometimes trading above or below 1.000.
As for the average user, the differences in price are only evident and a critical factor for trades that involve large sums exceeding 1000USD because you can not trade fractions of a dollar in Spot.