Why Obama Used Multiple Pens to Write a Single Signature?

Here are some pictures of President Barack Obama signing the historic healthcare bill at the White House. What may surprise you is the fact that Obama used not one, or two, but 22 different pens (see the top picture) to sign this history bill.
TIME magazine explains the rationale behind multi-pen signatures:
The pen used to sign [...]

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Why Obama Used Multiple Pens to Write a Single Signature?