Setanta wrote:It would appear that they may not be identical garments.
Well, basketball players wear sleaveless shirts too (no doubt they're polyester, not woolen), and those are called "jerseys." They're practically indistinguishable from Australian Rules Football "guernseys" (which, no doubt, are also made of polyester or some other synthetic material).
Essendon Bombers guernsey
Chicago Bulls jersey
I'm sure if Americans started playing footy, they'd be wearing jerseys rather than guernseys, just as
American rugby players wear jerseys.
Of course, calling a jersey or a guernsey a "jumper" is pretty funny for an American, since "jumpers" are one-piece items of wearing apparel for babies.