It's pretty clear at a glance that those letters weren't signed with an autopen. The signatures are slightly different.
For a good example, look at the "O" in Obama.
The signatures aren't xerox's, they were signed by auto-pen. You do understand what auto-pen is?
The letters are form letters signed from wherever Obama was when it was signed. Every letter was identical except the names of the addressees.
If they were signed with an auto-pen, the signatures would be identical. And of course they're form letters.