How can that be?
In this case, ABC admits it skewed its sampling of voters by splitting their sample percentages thusly: 37% Democrats, 27% Republicans, and 31% Independents.
The problem? The registered voter rolls do not match the sampling percentages used.
Of course, while democrats may enjoy a slight registration advantage of a couple of points, it is nowhere near the 9 points reflected in this latest poll.So if you heavily sample Democrats in urban areas (particularly poorer neighborhoods) and more lightly sample Republicans in more moderate suburban areas, the polls will skew heavily in the Democrats favor. The age of those surveyed can also heavily skew poll results.
Meanwhile, we also pointed out that with huge variances in preference across demographics one can easily "rig" a poll by over indexing to one group vs. another. As a quick example, the ABC / WaPo poll found that Hillary enjoys a 79-point advantage over Trump with black voters. Therefore, even a small "oversample" of black voters of 5% could swing the overall poll by 3 full points. Moreover, the pollsters don't provide data on the demographic mix of their polls which makes it impossible to "fact check" the bias...convenient.
If you want to get an eyeful of how polling can be influenced to give the results those hiring the pollsters want, Read The Whole Thing.
As the line from Don Henley's song Garden of Allah says, “I can get you any result you want. What's it worth to you?”