Large discrepancy in visitor numbers from google analytics plugins

Hi,

We use Yoast’s GA plugin to track just the traffic on our main site.
http://yoast.com/wordpress/google-analytics/
It currently shows around 15,000 unique visitors for just one site.

Our project also uses the wpmudev GA plugin to track our entire network.
http://premium.wpmudev.org/project/google-analytics-for-wordpress-mu-sitewide-and-single-blog-solution
It currently shows around 11,275 unique visitors for the entire network.

Any ideas what would cause such a large discrepancy?

Thanks,
Jeff
culture360.org