I don't understand the joke in today's strip. Since a pound is a unit of weight, not mass, measurements taken under identical conditions should always be the same by definition. The force that a certain mass of feathers would exert on a scale (assuming as Danny does identical conditions) would take into account buoyancy and thus, a pound of feathers and a pound of lead are by definition exerting the same amount of force on a scale.
Unless I'm misunderstanding US measurements, and a pound isn't a force measurement. A pound according to wikipedia is the amount of force exerted by 7000 "grains" (a mass measurement, about 65 milligrams) at sea level, notwithstanding local gravitational variations.
So a "pound" of feathers could be the amount of force exerted by 7000 "grains" of feathers, or 7000 "grains" of lead, under Earth-normal sea level conditions.
Taken that way, a "pound" could vary from negative values (7000 grains of helium) all the way up to the upper limit.
For today's strip's joke to work, then, the entire concept of "pound" as a stable unit of weight is gone and is entirely materials and conditions-dependent.