The data you see, and the data we collect, is a snapshot of your statistics.
If all works, we take at least 1 snapshot of your Twitter data a day. This might be in the morning, so every change to your Twitter account that happens after that moment will be collected the next day (and thus improperly attributed to that day). This is unfortunate and resolved only by a forced update, as you stated.
Our premium accounts actually offer statistics by the hour. We take a snapshot every hour (as opposed to one a day) and show this in graph. More information about the Premium services can be found here: http://twittercounter.com/pages/premium or you can contact me at firstname.lastname@example.org.