The price convert from coin to btc

Buy 4 coins standard BTCUSD PERPETUAL contract, when switching to BTC display, the quantity shows 0.0085, one day later, the quantity shows 0.0083, may I ask that I bought by the contract into the corresponding coins, how does this conversion price come?

Hello, I would advise consulting with a Customer Support via website’s chat to be able to explain to you with precise details from your account’s data.
Here, we can’t look into account’s data and our support is more on the API endpoints itself.
Thanks for understanding.

OK, thanks aisling.