Sometimes, one asset is quite low in the pool. If you are depositing coins that the pool is currently lacking, you can get an additional bonus. The main reason for this is that this asset is currently slightly more expensive so if you went to a centralised exchange you might sell it for a little bit higher. The deposit bonus reflects that. The other reason behind this is that the pools are always trying to balance themselves and go back to equal parts so depositing the coin with the lowest share will get you a deposit bonus. When you withdraw, the same principle applies (but reversed). If you withdraw the coin with the biggest share, you would get a bonus but you still choose what coin you want to withdraw.