I'm leaning towards thinking there might be an actual reason for this.
Feel free to correct me, but in the times I've worked around the food industry, restaurants are serving these products from systems hooked up to a water source from the local site (i.e., their own faucets.)
In other words, you're being served a mixture of flavored syrup, carbonation, and local tap water. Usually, water tastes a little different everywhere you go (even in your own house, the water from the kitchen can taste different from the water in your bathroom), so this is why the "same" flavor of pop might taste differently from place to place.
The canned products are not only mixed from a different water source, but all the ingredients are also compressed together inside an aluminum can (practically marinading, if you will) and have been sitting on the shelf for who knows how long. This will likely affect both the taste and the chemical makeup, and, I imagine, could possibly trigger a sensitivity.
Again, I could be wrong, but when you draw pop (soda) from a fountain machine, which is what restaurants have, the syrup is being mixed with the local water and carbonation as you draw it, so not all of the ingredients have been stewing inside a metal can for months.
I could definitely see this making a difference for some people.
In many ways, the product you buy on the shelf is not the same product as what you get in a restaurant.