I think the issue here may come down to terminology.

The physiological component here isn't really surprise, but conscious vs unconscious action. In short, as you start the action of letting go your body will automatically tense your muscles a bit in anticipation of the shock from releasing the bow string. Using a release device avoids that, you don't tense right before release because your muscles aren't aware that you are starting to release.

On a mental level you know to a pretty good degree of accuracy when the release is going to happen, but the exact moment is uncertain enough that you don't automatically tense in preparation for it.

Equal parts despair, distain, and disappointment.