The STS is right against the string. I think it works better because the arrow does not come off the string until the string is in it's most forward position with maximum energy transfer. Think of it like someone hitting a QB's arm before the follow through. If the arm is hit right at the middle of his throwing motion the ball comes out slower than it would at a typical release point. However, that does not explain why it is slower without the STS. Unless there is a sweet spot like qzq38b stated.

I'm going to run some more tests this weekend and will post the results with chrony pictures.