Is it possible that adding noise to a signal can increase detection possibility?
Our ears have a hearing threshold- i.e. a minimum amount of sound power that is required for them to detect.  Thus, a pure tone that is less than the threshold will not be detected.  If white noise is added to the tone such that the total power of the tone and noise exceeds the threshold, then the ear can hear the tone.

Perhaps you have some sort of similar non-linear effect going on.