^ Maybe because when he sings in usual way, his voice got so low like when he's singing Orange Caramel's Magic Girl with Jjong. It was so bass.
EDIT
Now that you mention it, I went YT-ing. I found these.
Spoiler:
His "Juliette, oo~" and his english sounds smexy
I forgot to tell you about His Usher's OMG Solo on concert lol
He sounds good here
the female guests are mesmerized