I see a profit but my Bitcoin (BTC) balance goes down - how come?

The trading bot takes into account only a deal's profit, it doesn't count profit/loss caused by the change of your coins' price. I.e. you have Bitcoin on your account, the bot made you a 10-USDT profit, but BTC dropped in price to USDT, and you have lost 11 USD just by having BTC on your account. So you have lost $1 at this point.

Another possible reason is an increase in Bitcoin price.

Example: