The difference in trading fees between BNB-USDB on PancakeSwap and Beefy can be attributed to the way each platform calculates trading fees. PancakeSwap charges a flat rate of 18% for all trades, while Beefy charges a variable rate based on the volume of the trade. This means that a larger trade will have a lower fee than a smaller one. Additionally, PancakeSwap charges a fee for each trade, while Beefy only charges a fee for the first trade. This could also contribute to the difference in fees between the two platforms. Ultimately, it is important to consider the fee structure of each platform when deciding which one to use for trading.