2 things:
1. the difference could merely be statistical. Given the MOEs in each poll, they are pretty close to each other.
2. Zogby and WP are not reverse of each other. Zogby is showing Bush slowly gaining, while the WP shows a tied race. The opposite of Zogby would be a Kerry lead which the WP does not show.
Don't forget we got Rasmussen sort of backing up Zogby and saying undecideds are coming our way