Hidden Users?

First, disable all of your plugins and switch to the default theme to eliminate the possibility that something is messing up those counts. I have seen this before and I think it was a plugin causing it, but that might not be true in your case. If you still get oddball counts I’d worry a little.

I wouldn’t recommend bulk emailing through WP even if you could find a plugin to do so. Sending 2260 emails from your domain all at once is a good way to trigger spam filters. I speak from experience. It happens to us about every two or three months (almost always with Hotmail and affiliates) and that is sending maybe a hundred or two per day, not two thousand at once. There are online ‘newsletter’ services that will let you avoid that problem. It is pretty easy to download the email column from the users table into an excel spreadsheet and upload to a newsletter service.