c# - Twilio total call duration does not match with billing minutes -

in attached image , total voice minutes july 30 minutes. if pull call logs same month july 2014 (using instruction in https://www.twilio.com/docs/api/rest/call) , total duration 17 minutes. shouldn't value of usage , total call duration in log equal ?.

here test source code finding call log files month july 2014. appreciated.

       public static void  calllogs(string accountsid, string authtoken)        {          var twilio = new twiliorestclient(accountsid, authtoken);          var request = new calllistrequest();         request.starttimecomparison = comparisontype.greaterthanorequalto;         request.starttime = new datetime(2014, 07, 01);                  request.endtimecomparison = comparisontype.lessthanorequalto;         request.endtime = new datetime(2014, 07, 31);         var calls = twilio.listcalls(request);          int? voiceminutes = 0;         decimal? totalcost = 0;         foreach (var call in calls.calls)         {              if ( call.price != null)             {                 voiceminutes += call.duration;                 totalcost += call.price ;             }              console.writeline(call.price  +"-" + call.datecreated + "-" + call.from + "-" + call.to + "-" + call.status + "-" + call.duration  );         }          console.writeline("total voice:" + int.parse ((voiceminutes/60).tostring() ));         console.writeline("total cost :" +  totalcost);     } 

for billing minutes, twilio round calls nearest minute. should same. this:

voiceminutes += (call.duration + 60)/ 60; 

and then:

console.writeline("total voice:" + int.parse ((voiceminutes).tostring() ));