Because pollsters are not as impartial as you would want them to be, as Sam Stein reports for the Huffington Post.
Incidentally, I am not pointing fingers to one side or another: both sides in Stein's example are equally beholden to special interest. "McInturff [the Republican pollster] has done extensive work on behalf of America's Health Insurance Plans (AHIP) -- a major reform opponent." And, "Hart Research [the Democratic counterpart of the poll] has done work on behalf of a host of unions who support the public option, including the AFL-CIO, American Federation of State, County, & Municipal Employees, and the Service Employees International Union."
Sam Stein writes that Democratic pollster Mark Blumenthal commented that "[b]oth McIntruff and Hart are always hugely conflicted in terms of partisan clients but almost always in opposite directions," but that "[the] long standing assumption is that each serves as a check on the other." Really? Do two wrongs make a poll right? Good grief!