what mcluva420 said is correct. It's a video effect to make the video seem like it is in slow motion while the mouth is in sync with the lyrics. Hard to explain... but here's an example:
the making of hikki's Be My Last, watch it from 3:05
then compare it with the real PV, you'll see that this scene seems like it's been slowed down, but the mouth is still in sync with the music playing.