That is why I think his voting samples are so messed up. The number keeps defaulting to the -17 area. I believe that is the true number.
That seems to be his base -
One needs to consider that this graph is prone to jitter with the way the score is tabulated by using two related, but not perfectly correlating data sets.
To explain -
Say Obama does something that makes one side happy, and the other side less angry. He will get a bump from the happy folks, and then a second bump from the less angry folks - this results in the wild swings that these numbers are prone to. And, of course the reverse is true.
With the exception of Obama's big speech - each line of this graph has been fairly consistent, even though the numbers have been all over the place.