Anything that makes the vocal cords uneven, such as in mass or tension, can put them out of sync. With a mild asymmetry, they may oscillate out of phase. Under a strobe light, it will look like they are chasing each other. They may never touch each other and so, effectively, even while they are crossing each other’s path in the midline, they do so at separate times, so air continuously leaks out. This is a gap created by timing.