Bills usually have a stigmata attached to them. Even if the main section of the bill is in alignment with their political beliefs, there could be a passage that states funding of a certain department would be increased/slashed that would have absolutely nothing to do with the bill in the first place.

Unless you look at every single bill along with their voting record, it's hard to say that Obama was a bad senator because he refused to vote 117 times. Much of the time, bills are political traps.