I hate to be "that guy" but since it is only a theroy to explain what they don't understand, it's weight is just an educated guess. Could weigh much more or may not exist at all....we KNOW black holes exist, but again, everything about them is speculation.
Maybe they didn't want to think about unit volumes they would use in future. This is not the only one thing, which is unlogical for the first sight so why concentrate only on this one?
"each amount, which would normally weigh 1 pound" There is no standardized size for 1 pound of an undefined substance.
Different materials have different densities, the density defines the size of 1 pound of that material.
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It's like the riddle: Which weighs more, 1 pound of Lead or 1 pound of Feathers?
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The answer, of course, is that they both weigh 1 pound, despite the fact that 1 pound of lead would be a very small ball, whilst 1 pound of feathers would be a medium pile.
http://i.ytimg.com/vi/XkHdgMOuuBs/hqdefault.jpg
Different materials have different densities, the density defines the size of 1 pound of that material.
.
It's like the riddle: Which weighs more, 1 pound of Lead or 1 pound of Feathers?
.
The answer, of course, is that they both weigh 1 pound, despite the fact that 1 pound of lead would be a very small ball, whilst 1 pound of feathers would be a medium pile.