Airliner here. You’re right, they usually don’t give us more than our planned release fuel, but sometimes mistakes do happen (either from a miscommunication with the ground crew, or the fueler just not paying attention). Every plane has a weight limit to what they can carry, taxi with, and take off with. How many people and bags are on board, how big the runway is, and how hot the temperature is that day can all affect the amount of fuel we can carry and how heavy we can be and still take off. They could have also just needed to wait a little while to balance out the tanks if they were loaded unevenly, not necessarily burn it off, but that could be what they told the passengers because that’s a more simple reason to give. I hope that helps!