Someone told me that it wasn't the signature that was important, somewhere there is a camera that grabs a picture at some point in the signature process and signing is more or less supposed to ensure you are in a position to be captured by the camera.
I don't know if it is in the card reader itself or somewhere near the register or if it was unique to this one retailer where I was told. I had kids shut a car door on my writing hand and couldn't move my first three fingers for a couple days so I was signing off hand and it looked nothing like a real signature. I just made a squiggly line once and it accepted it then made a comment about it. The clerk who was also a manager at the place told me about the photo thing. He could have just been blowing smoke too but wasn't too concerned. It might be because I'm there regularly on my way to work too.