How does AT&T Prepaid phone (Go Phone) count phone connection time?

Hi, I have a question. I found my talking time is far less than the records in AT&T for each call.

For example, a call duration is 35 sec (i phone) but on AT&T history is 1 min 10sec. Another call is 51 sec on i phone but still 1 min 10 sec in ATT history. the 3rd one 55 sec on i phone but 1 min 11 sec on ATT history.

Looks to me ATT Bill charged me too much. Can you answer why the charged seconds are neither accurate nor precise?

I am suprised to have this finding after so many years with Go-Phone when I start to use car blue tooth so I can count each call.