A suit made from a single photo fits only in that one pose.
An example. The strategy produced ten losses. You add: “don't trade on Tuesdays”, “RSI no higher than 63”, “only if the previous candle is green”. Each rule removes a couple of losses, and the past now looks beautiful. But these rules have no reason behind them: Tuesday is no worse than Wednesday. You simply fitted the rules to past cases. Traders call this curve fitting.