Hey Calum - that's actually the number from WordPress.org. For the first day or so when they made the recent change to show active installs on the plugin repository, they were showing the actual number (rounded down to the nearest 100,000) for the 1 million+ plugins like they do for those below 1 million. NextGEN was showing 1,300,000+.
Now, they're just showing "1 million +" for any/all plugins that have more than 1 million installs. Doesn't matter if you have 1,000,001 million or 2 million or more.