I am also confused as it is not clear to me where did you notice that ABANDONED call. The base FOP2 does not report statistics, you have two plugins that can show stats, and they behave differently as they source the data from different places.. the queue stats plugin shows the stats reported by Asterisk queue application (you can contrast that data with the command asterisk -rx "queue show"). And the full wallboard, that uses Asternic CCStats PRO repots.
They are different sources, and in this case ABANDON has a different meaning.
Asterisk Queue application will count any call that enters a queue and is not connected to an agent as an ABANDON.
In the other hand, Asternic Call Center Stats uses the asterisk queue_log file for its source data, and discerns between ABANDON event in that log (a caller enters a queue and hangs up before being connected to an agent), and Unanswered calls, those are calls that enter the queue, that are NOT connected to an agent, but diverted to somewhere else due to either pressing a key, a timeout, or the queue become empty.
Your last question does not make sense to me: Why a call is calculated as abandonded before it was answered? Because an ABANDON is exactly that, a call that was not answered by an agent (but entered the queue). So either if you use asterisk queue application abandon stats, or the queue_log stats, an ABANDON will always be recorded if a call is not connected to an agent (thus answered).
Now, if you can be more clear about the exact numbers you saw, the full line in the report, and perhaps contrast that with the line in /var/log/asterisk/queue_log, then I can give you a more specific answer.