We know that SuperTx is consisted by three LP reserving pools,the exchange price is also priced according to the number of assets in the pool according to a certain algorithm, so when the price fluctuates, there will be some arbitrage opportunities.
For example, we deposit 1,000 DAI and 1,000 USDC, the exchange rate is 1:1. Now external DAI reaches 1.1, If the price of the reserve pool does not change then there is an arbitrage opportunity. Arbitragers could buy USDC in the external market and exchange it into DAI in SuperTx, and then exchange it to USDC in the external market, they could get 10% profit.
If we withdrawal all assets, we will finally get 950 DAI and 1050 USDC, now the DAI is 1.1 ，if we don't consider trading fees, we get loss because of the profits of arbitragers. The total DAI amount decreases but the USDC increases and the total number remains the same.
If external DAI/USDC returns to 1:1, arbitragers can make profits in the opposite direction to keep the balance of the reserve pool.