by dirty fingers » Sun Nov 17, 2013 4:54 am
In 'machine doubling', the coins is only struck 1 time. However, the die has a little jitter to it. As the actual strike is taking place, the die 'snaps into place' at the last moment. This is believed to be a loose fitting die.
As to the reason why they have less (no) extra value, I really do not know, other than they are likely much more common. I think one reason why the double die varieties have such a premium on them, is that they are actually something human done in the process, and they are things that SHOULD have been inspected (the finished die), and never been used at all. The machine doubling seems to be a 'normal' mechanical side effect of the minting process, something the mint would like to not have happen, but it does, and there is no way they can remove the examples from the multi billion cents minted each year.
The pictures on the example page linked above are good examples of machine doubling. I have both examples of the 68s, the same 69s, 2 of the 72's and about 20 other machine double examples.
I always take life with a grain of salt, ...plus a slice of lime, ...and a shot of tequila.