Sometimes you want to sell at exactly $55.5, not higher not lower.
If your FA/broker sells lower than you asked, then you are obviously unhappy.
If you get sold higher than you planned, you may regret too, because you bought at $57! You wish for the price to recover. You have a max loss tolerance (and a stop-loss order) so you want to wait till $55.5 and then give up. In this case you gave up too early. If you repeat this frequently, you are a chicken.
Now turn to buying….
Q: As a buyer, when would you regret buying lower than your target of $55.5?
A: say you tell broker to buy at $55.5 if market condition remains calm. Earthquake! Broker bought at $27, half your target price. You fire Broker because he didn't know how to judge “market condition”.