You may want to look in to that one before you post it. The voting in Congress is is Yea/Nay/or not voting. As far as I am aware, it is not possible to vote "present" in the US Congress.
What you are referring to is his voting record in the IL senate. The practice of voting Present was widely used by all members to indicate that they would vote for the bill with some changes as well as other reasons.
"Present" article
This article gives a decent explanation of what is at issue. Some of it was politically motivated, some of it was based on genuine concerns. Given the fact that he cast over 4,000 votes, 136 present votes amounts to just over 3%. And as indicated by the article, it is part of how they play in the IL state senate.