Here's an interesting article on Guinness from beveragebusiness.com,
link
An excerpt...
"There was one more question in that set, about whether the beer in the can is the same as the beer in the keg. It is, but it's not the same beer as is in the bottle. If you look at the labels on the can and the bottle, you'll see that while the can contains Guinness Stout (as does the keg), the bottle contains Guinness Extra Stout. That's not just marketing hype, that's a very different beer. It has half again as much alcohol, a much fuller body, and a lot more hops, exactly what you'd expect in a beer called Extra Stout.
There are actually more than fifteen different variations of stout under the Guinness name in various parts of the world, all the way up to the heavyweight Guinness Foreign Extra Stout. This monster 7.5 percent alcohol stout is brewed specially strong for the tropics, where a beer needs to be big enough to take care of itself in the hot weather. But most Massachusetts customers are only interested in two kinds of Guinness; the Guinness Stout they get here and the Guinness Stout they can get in Ireland. "It's different," they say, "the Irish keep the good stuff over there! It's stronger, it's smoother, it's better!" Some even believe "American Guinness" is brewed in this country."