The McDonald’s website says that “in order to ensure our drinks are always meeting a gold standard, we have proper filtration methods in place.” In other words, better equipment gives them a huge advantage over the competition, especially when it comes to water quality and temperature. They literally ultra-filter their water twice before mixing it with anything.
Plus, McDonald’s sells a high volume of soda. This means that the syrup does not sit around for very long, unlike at other restaurants. Most McDonald’s sell so much in fact, that their syrup does not come in boxes like most franchises.