Hello Miko.......
Production freaks are very common. Things just don't always line up the way they are supposed to. Over-sized or undersized stamps can be caused by the perfing machinery being out of alignment or the paper can be shifted out of place. After being perfed sheets are cut to pane size, this can also cause size differences.


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Color differences can be caused by the use of separate batches of ink or in some cases if ink barrels sit too long the ink can begin to separate into layers. (like oil & water)


When colors are applied in separate layers, misalignment causes multiple & blurred images.

Changelings are a different type of item, this happens after production. Chemicals of many types can affect colors. This blue stamp has had the red removed.

For all of stamp collecting freaks have been treated generally as trash and were things to be avoided.....items used as give aways for children. It is only fairly recently that some people have actually started paying for such things.
It is important to understand that while these oddities are curious and interesting, they have no real added value, in all but a few cases. The US C6 "grounded" Bi-plane is one that some people are willing to pay for, price or value is determined by competition for the item, not because it's worth more.
For most stamps over-sized stamps.........equally over-sized margins can have an added value, one or two large margins is just a poorly centered stamp and is worth less than an average stamp with normal but even margins.