spark plug lead shielding

[ralt]

Hi all.
I recently replaced my spark plug leads from factory to NGK. My timing light would not work (no light) with the NGK leads but had worked with the factory leads. Oh no the timing light is broken. I consulted the trouble shooting guide for the light and it stated you may not get light if two leads are close together. So removed the lead from the clips and away from the other leads and I had light. It would seem the factory leads are shielded better than the aftermarket leads. My question is what causes this to happen?