ayu instrumentals

[mirrorcleworld]

how come the instrumentals on ayu's singles have lapses of ayu's voice still on them? i find it really annoying most of the time since i enjoy instrumentals a lot.
i've also noticed that some other artist's "original karaoke's" and "instrumentals" also have these leftover voices, like hikki's singles for example.

but with other's like namie's, aoyama thelma's, and SoulJa's (i not sure you've ever heard of him, -koko ni iru yo anyone?-) instrumentals have no voices popping out at random whatsoever, they only have them VERY RARELY like only 1 out of a 100 or something (i'm not sure)

so does anybody here know why is that?