I noticed this a few weeks ago too but didn't think too much of it.
In fact, a lot of the scatter bonuses in RTG have similar flaws. If you ever play Tiger Treasure, the bonus is activated by 3 consecutive scatters either left-to-right or right-to-left.
So if is no scatter comes in on reel 1, your only option is to get it from right-to-left (either reels 3,4,5 or 2,3,4,5).
When you see no scatter on reel 1 and reel 2 drops in with a scatter but with no sound, you aren't going to get the bonus. Yet if on that same spin, a scatter appears on reel 3, you will hear the sound and the reels will pause a bit... the problem is, since there was already a scatter on reel 2 you can't possibly get the 3,4,5 combo - you could only get the 2,3,4,5 combo. And if you were going to get the 2,3,4,5 combo, then the sound would have been made when reel 2 stopped spinning, and it would have paused before reel 3 showed itself. But it will still try to make you think you have a chance if a scatter drops in on reel 3.
I hope this makes sense? It's basically the same thing, but with a slightly different setup.