Identifying people that are high volume has been something that has taken the longest time to really settle down.
I'm getting much happier with this latest (the 3rd? 4th?) incarnation.
If you remember, the previous version simply identified anyone that tweeted on average more than 50 times in a day. That was much more reliable than earlier versions, but suffered from one major limitation. There's a huge difference between someone who tweets "hello world!" in the morning, then uses the other 49 tweets to chat to their friends, vs someone that just blasts out junk 50 times a day.
The difference is - how many of those tweets are public?
Why are high volume tweeters even a problem? Well, this is something that people tend to forget once they start following more than a few thousand people. When you're following that many people, there are so many tweets flying past it's mostly a blur. So, if you tweet like crazy who cares, it gets lost in the blur, right?
What is forgotten is this: Of the active users on twitter, most people only follow 2-400 others. One high volume user can flood an entire tweetstream, making it impossible to connect with anyone else.
With that in mind, the new high volume algorithm works like this. If you tweet publicly (ie, anything other than a reply) more than 24 times a day, you're listed as high volume. This isn't completely accurate, since if you have any overlapping friends, you'll see their conversations with them too, but it's a good estimate.
Once an hour may not sound like a lot, but once you factor in work, sleep, play - oh, and the fact that this is only public tweets, it's an absolute ton. So talk, talk away! Just connect, make some friends! Don't blather on about yourself all day :)