question

It's likely to either be due to the system you're running on or the database. The system you're running on may have other processes running that can lead to contention on resources such as the disk or cpu or memory. That will lead to differences, even wide differences, between run times. You should be monitoring the server while you run the replay which will enable you to determine if you're seeing differences in CPU or memory, etc., queuing, or unusual waits. The database could be causing the issue if you're not starting from a common restored state each time. This is because you may be seeing variations in statistics which can lead to variations in execution plans which certainly cause changes in performance. You may also be seeing page splits or something along those lines during subsequent runs that you didn't see on previous ones (or even page splits on early runs, not seen on later ones) that will seriously impact performance. Be sure you're starting from a known state each and every time you do the run. But, the real key to the whole thing is to have monitoring in place so you understand what is happening during the runs.