Pollsters are businessmen who must sell their product to survive.
There are fundamentally two types of political polls. One is as valid as possible in order to allow the candidate to a) focus on the most productive issues and b) direct resources to those states where the electoral college "bang for the buck" is greatest. These polls must be as accurate as possible. Rarely are the results of these publicized, partially because the data they has utility to the opposition as well.
The second type of poll is for public consumption. It is designed to rally the faithful (or discourage the opposition). These polls are the least credible as they are not in the service of objective truth but are to advance the objectives of the sponsor.