Two answers. (1) That number isn't as well known as you might think, if you're referring to the dem/rep/ind breakdown of the population; the number changes based on economic conditions and other news and cannot be assumed static. (2) The samples are random, and fluctuations in those numbers should be expected with random samples - part of the reason for the +/- 3% uncertainty bounds normally reported for a poll of 1000 people. Some pollsters adjust for the difference between their sample demographics and the population, but that runs the risk of missing the fact that party identification is actually changing. Others intentionally slant their samples, but the sleaze in some polling is not the only reason for these results.
I also understand that a sample of 1000 yields an uncertainty of +/- 3.16% but that does not change the fact that Others intentionally slant their samples, but the sleaze in some polling is not the only reason for these results."
Maybe I'm paranoid but I sometimes feel the media attempts to manipulate rather than report.