I expect this is likely an issue of timing resolution on your system. e.g., on any given system it take X amount of time to process the sound coming in and to check it to see if the amplitude is high enough to qualify as an RT. This cycle repeats over and over at a very fast rate. When you have final digits in the RT that seem to repeat, that usually indicates that the standard cycle is reliably occurring in "chunks", e.g., every 10ms rather than every millisecond. The best way to ensure the validity of your voice RTs is to check a random sample of them manually. This can be done fairly easily with a freeware program called Audacity which allows you to look at the amplitude of wave files on a millisecond scale. If you want to send me half a dozen of your .wav files, I can show you. Doing so, I expect, will show that you have a reliable ~10ms timing resolution on your RT detection.