Binance isolate margin fee why can't I use bnb to pay the fee

Can someone please answer this question.. I cannot find any info on this anywhere ! So Binance has a trading fee of 0.1% on every single trade ( buy and sell ) + interest when margin trading. Now my question is, in the case that I Margin trade (isolated 10x) the trading fee is 0.1% right.but when u use bnb it comes to 0.075%(is it correct).so I clicked the pay by bnb option.but in my trade history it comes as 0.1% as the fee.and in my fee level option they say no bnb available although I have clicked that option for the cross margin.iam trading in isolated margin.