I posted a query on shareThis forum and got their response as
Hi,
ShareThis uses its own mechanism to calculate shares and clickbacks.
When somebody clicks on the ShareThis button, a count call has been initiated and the count on the buttons increases at the same moment, it is the basic functionality of the ShareThis buttons. Here, the user has an intent of sharing the data and hence he has clicked the button, but if he cancels the sharing, the counter never goes back as the count call which is initiated earlier has already being tracked by the Analytics Dashboard.
Hope this clarifies.
If you wish to implement native counter buttons (available right now for Facebook, LinkedIn and StumbleUpon), you can make changes in your ShareThis code as shown below -
stLight.options({ publisher: 'PUBKEY', nativeCount: true });