Why PnL is different?

As you are using different brokers for master and slave the spread/price/slippage could be different leading to different execution price and PnL, mainly if the trades are executed with a market order. Also as trades are copied from one account to another account there is a small latency that could lead to a different PnL. This cannot be avoided. Using a server location the closest to your slave broker could decrease the latency and minimize these consequences.