I am not sure how a call would show up that is abandoned while actually ringing on the Agent's phone. I do use the abandoned calls report, and I can see on mine that the most calls are abandoned within the first 20 seconds, and since my initial greeting is 25 sec long I know they didn't even to bother to wait till the greeting is over. I work in a healthcare system, and nurses are some of the most impatient people I know - they would rather hang up and call back 10 times than sit in the queue for a minute. However I have had problems with mys system and have watched the realtime status graphs, and I have seen more than a fair share of people in queue where I am guessing the handset was on its way back to the cradle just as the call was ringing on the agent's phone. I am not sure if that is an instance where it would show up as abandoned or a missed call. It will definitely show up as an "offered" call, but I never looked close enough at the rest.
The other question I would have is if you have your groups set up to overflow to another group or an extension. I don't know what it is considered as from the leaving standpoint, but I know on the incoming group it will show up as a call offered as overflow rather than primary.
The other thing I have noticed over the years is that the numbers don't add up sometimes. If you use 5 or 6 different reports and look close enough you might see the number of calls that were offered to the group in a day, and if you add up the number of calls handled and the number abandoned it doesn't add up to the same number as the system says calls were offered - there are usually a few missing calls for me that I have no idea where they went - I don't have any overflow set up. You either answer the call or you don't.