Say for Example, you execute a script and get the total elapsed time as 0:01:57 milliseconds which means 1 minute 50 seconds, but when you added the response time of all the pages of this script it displays as 12107 milliseconds which means 12 seconds.
Below is one such explanation of some of the other factors that make up the total elapsed time.
1. At the end of each page in the test log you'll see a "RTAA" value which is negative. This is an adjustment factor that RPT subtracts from the observed pages' elapsed time to give a more accurate elapse time for the page. In the below snapshot, The response time is being reported as 4,976 ms with an RTAA of -290 ms. So there's 29/100th of a second accounted for in the total elapsed time vs adding up all the individual response times
In the below snapshot, before the first request of the first page is executed it shows when the execution actually started. In this example it started at 1109 ms. This is overhead and accounts for some of the other missing time
The third and easiest time to factor is the think time per as shown below.
Also, when running a schedule of multiple users, when adding up the individual page response times, use the maximum response time, not the average when doing the calculations.