You're sipping slightly ice-diluted Coke with a fresher, higher CO2 mix and specific and fresher syrup storage from the bottom of the McDonald's Coke with a straw. it's more watered down as the ice melts if you sipped from the top.
Drinking it out of the can is drinking Coke as Coke intended; however, my theory is that pouring canned over ice may dilute it, but it cools it and the CO2 evaporation rate down lower than the mixed McDonald's coke, making the McD's coke tastes bubblier and fresher. Also, McD's water filtration system versus the frozen gravy cubes you're using.
Drinking it out of the can is drinking Coke as Coke intended; however, my theory is that pouring canned over ice may dilute it, but it cools it and the CO2 evaporation rate down lower than the mixed McDonald's coke, making the McD's coke tastes bubblier and fresher. Also, McD's water filtration system versus the frozen gravy cubes you're using.