I seriously doubt stove manufacturers are required to measure the heat output of a burner - they just calculate it. And since BTU numbers are important to the unenlightened (those who don't use alcohol stoves), I suspect that manufacturers put the most positive spin on them.

Obviously you can burn a lot of fuel under the right conditions, but will you encounter those conditions on the trail?

The older Colemans were jetted such that they would blow themselves out if you pumped it too much. And I know that they wouldn't boil a quart in 4 minutes in a typical saucepan when it was cold out. But they got the job done and lasted forever.