Perhaps you did not take into consideration that there is a One Time Only activation fee on zkSync, which you do not have to pay again on subsequent transactions after the first one. I.e. after the initial transaction, fees should be similar between L2 → L1 withdrawals vs L1 → L1 transactions, and transactions L2 → L2 will be actually much cheaper, which will be an important consideration once the L2 deposits are activated on exchanges such as OKex.