It's all in the wording of the question.
Back in 1996 and only 27% favored gay marriage the question read "Do you think marriages between homosexuals should be legal?"
Now, the question in worded much more softly to the point to where there could be confusion."
The question should read:
Are you willing to stand-up against Biblical Law and for what is evil and destructive to Christian society and ultimately face the consequences on Judgement Day and support marriage based on Sodomy?"
Their poll numbers would then be quite different.