DoCoMo — Docomo cuts more then our call's actual duration

It's been from long time. If Docomo users have been noticed or not but I have always noticed this, When I call some one, & at the end on my mobile screen the call duration is 29 sec. But when balance message comes at the end of call, it always 1-3 seconds more then the actual call duration that shows on my screen. I had companied few times about this in Docomo care but they say we calculate the cost of complete call., at end at our server or some thing like that... So it's always 1-3 seconds more. Let me know in comments if other Docomo users had same issue. I say if that's the case then Airtel, Telenor/Uninor etc all had same problem but it's not there. So that reply from Docomo care is not satisfactory to me. I say if the servers are slow then make them faster, why are billing for your slow servers or other problem's cost from customers... I like docomo as Docomo to Docomo rates were less. But then added Docomo user must be part of Buddy net. So I think new Docomo users are not covered in it. So again more bills to customers...