Over the course of a modern day player's career, they may sign hundreds of thousands of autographs.
Between signing in-person at the stadiums, through the mail, charity events, and of course - baseball cards, there are many times a player may be requested to sign something.
Throughout the years some players' signatures evolve. Many contributing factors may explain this evolution.
On the right you will see an example of Scott Rolen's signature over the years. From 1995 until 2008+, his signature has changed a few times; looking completely different now than when he first started signing baseball cards.