I've done a little more investigation, from the WordPress Codex (http://codex.wordpress.org/Creating_Tables_with_Plugins):
If you're developing for a version of WordPress older than 2.0, you'll need to use the $table_prefix global variable, which is deprecated in version 2.1.
The next release of WP Statistics will remove $table_prefix and replace it with $wpdb->prefix in all instances.
I don't know if that will fix your problem or make it worse, but it will at least be correct :)