Here is the direct link to Pew's poll report (rather than just the Pew home page, which will change in a day or two):

http://www.people-press.org/2012/09/19/obama-ahead-with-stronger-support...

Pollsters use different methodologies, Richard. They have different sampling methods, they have different predictions about what the electorate will look like on election day so they poll different proportions of people. And then there's randomness built right into polling. Hence, "margin of error." A 5% lead means, "If our model is right, Obama is ahead between 2% and 8%. An 8% lead means, "If our model is right, Obama is ahead between 4.5% and 11.5%"

Poll averages and prospective electoral maps also differ, because they include different polls, weight specific polls differently (How much weight to give an everyday tracking poll? How much to give other polls?) and so on. The electoral maps have to decide how far ahead a candidate has to be to move a state's votes out of the "tossup" column.

Obama’s Lead Looks Stronger in Polls That Include Cellphones
By Nate Silver, Five-Thirty-Eight @ New York Times, September 19, 2012, 5:24 pm

[....] We keep track of which polling firms include cellphones in their samples and which do not. So on Monday night, I decided to run two alternate versions of the FiveThirtyEight forecast. (Note that all results are based on polls that were in our database as of Monday night, and so will not include Tuesday morning’s New York Times polls or others published on Tuesday.) [....]

Cell Phones and Election Polls: An Update
Pew Research Center Publications, October 13, 2010

[....] It is possible to estimate the size of this potential bias. The Pew Research Center for the People & the Press conducts surveys with samples of landline and cell phones, which allow for comparisons of findings from combined landline and cell interviews with those only from landline interviews. Data from Pew Research Center polling this year suggest that the bias is as large, and potentially even larger, than it was in 2008 (See "Calling Cell Phones in '08 Pre-Election Polls," Dec. 18, 2008) [....]