Acrobat remembers the signature style you last used and starts with it the next time. Apparently, at some point you changed signing style to digital signature (either intentionally or not and you may not even remember that you did that). You can change that. Right to the "Place Signature" button there is a small triangle. Click on it and it will pop up a drop-down list of choices. One of them is "Change" (I do not have Acrobat XI at hand to check the exact wording). Click it and you'll get the selector for the signature style.