I hear what he's talking about, it's called harmonic interference. If you've ever been in a band or orchestra you're very familiar with the effect during tuning when somebody's out of tune and they get closer or farther away from the group. In this case it's the result of multiple layers of voices. If they were exactly the same, it wouldn't sound like multiple voices, it would sound like one voice with the volume turned up. You need the voices to be different enough that you can actually hear them distinctly. However, because they are computer-based samples, you sometimes get periodic interference patterns like that, where real voices would instead have much more random variation, so the interference wouldn't manifest in that audible pulsing. Or maybe it's just reverb. I don't know, I'm tired and I'm not an audio engineer. Long story short, it's there, but it's not really something that needs to be "fixed".