The Tweet button counts the number of Tweets that have been Tweeted with the exact URL specified in the button. This count does not reflect the impact on Twitter of conversation about your content — it doesn’t count replies, quote Tweets, variants of your URLs, nor does it reflect the fact that some people Tweeting these URLs might have many more followers than others.
The count was built in a time where the only button on the web was from Twitter. Today, it’s most commonly placed among a number of other share buttons, few of which have counts.
For some odd reason, this change has caused many people to have their panties in a bunch.