Here's an example. According to a newly released Zogby poll, the Wisconsin governor's race is a dead heat -- Democratic Gov. Jim Doyle up only by 1% over GOP nominee Mark Green, well within the margin of error. But a Wisconsin Public Radio poll gives Doyle a comfortable 13% lead.
Both of the Wisconsin polls were of "likely voters" and the dates of each poll are virtually identical. The gap may reflect differences in how each pollster defined "likely voters." But who knows?