Average agent time reply - Zoho Desk

Average agent time reply - Zoho Desk

Hello, 
We are building a report where we would like to see the average time that an agent takes to reply to their customers. 
In Analytics we were using the measure "tiempo total de respuesta en horario laboral", but it does not show the correct values for some tickets, others seem to be ok. I am not able to find how this is calculate to understand why it might be wrong for some tickets. 
Thanks,