The problem with swaps on the destination chain is that we have to craft the whole payload in the beginning and send it with the source transaction.
Therefore, the amounts in the destination swaps have to be hard coded. During the first swap and the bridge transaction, the amounts can vary slightly due to slippage. To make sure the swap on the destination chain will work, we use the minimum amount the bridge transaction will yield for the swap to ensure it will work in any case.
That's why the amount is lower than the expected outcome of the bridge step.
The remaining funds that are not swapped are transferred to the user unswapped.
The relatively high difference in this example here indicates relatively high slippage on Stargate: