The main issue is still the devaluation of USDT. Relatively speaking, USDC is used less in China and Asia. When the market heavily sells off USDT, the price of USDT against fiat currency will show a negative premium, while USDC remains pegged to 1 USD, leading to a negative premium of USDT against USDC. The recent devaluation of USDT includes factors like S&P downgrading Tether's rating to 'Weak' and regulatory pressure from China.