Why is the USDT-USDC pool not 1:1 in ratio of assets?

The USDT-USDC dynamic pool is created as a stable pool that assumes both token assets are pegged to 1 which should be true as long as both USDT and USDC are remain fully backed and not depegged.

The ratio of assets does not have to be 1:1 (50/50) as the two asset types in the pool are supposed to be worth the same in the first place, so the price of either asset is not determined by the ratio of assets.

If the pool tried to maintain a 1:1 balance then any trade that further unbalanced it would cost more. This would make trading expensive and so would attract little volume and result in few fees for LPs. The live ratio between the two assets is a reflection of the market demand for buying/selling the two assets.

And the AMP factor controls how concentrated the liquidity is in the stable pool, which relates to how far the ratio of 1:1 the assets will go before it starts to charge more for trades that further unbalance the ratio and charge less for trades that move the ratio of assets back to 1:1.

If you want to deposit certain amounts into the pool other than the default pool ratio, you can turn off balanced deposit and deposit the specific amount you want.

1 USDT-USDC is the LP token of the pool and e.g. 1 USDT-USDC ≈ 1.06 USD is the virtual price of the pool.