Response durations may change due to Answering Machine Detection and the amount of background noise during the call. For example a loud TV, screaming children, or even a barking dog at the location of the recipient, can cause fluctuations in sound that may cause small delays in when your message plays.
Recipients must say "hello" clearly, for your message to play immediately. Users should always say "hello" when answering test calls yourself. Rogue background sounds, delays in saying hello, or even saying hello multiple times, may cause small variances in when your message begins to play. This will cause your overall call duration to increase or decrease by a few seconds.
For more information on how AMD works, please read:
http://www.callfire.com/blog/2008/09/19/faq-how-does-answering-machine-detection-work/
To turn off AMD completely, choose the last campaign type under Voice Broadcast, entitled: "No Detection - With Transfer".