When scrubbing, sound is heard before the waveform

I've recorded a narration and I'm breaking it down into sentences, stored as separate files. When I'm setting a marker just before the start of a sentence, by scrubbing, I've noticed that the sound is heard ~20 ms before the waveform. Meaning: I move the playhead to just before the sentence, then scrub backwards and forwards until I've found a suitable spot for the marker. When I scrub forwards, the sound always comes before the waveform indicates the sound should start.

Same thing happens if I scrub backwards to the end of the previous sentence: the sound is heard before the waveform indicates it should be.

I'm pretty sure I've noticed the same effect in Premiere.

Is this normal? Or is it a problem with my OSX or Audition?

I'm running Audition 5.0.2 under OSX 10.6.8.