What could be the problem if the exhaust on my car backfires after ive reved it in neutral.?

I have a 2000 Holden VS commodore UTE, it had a little bit of a miss while driving, so i replaced spark plugs and put in expensive new bosch ignition leads. At first i thought i may have linked up the wrong cylinder with the coil pack but after checking over a numbered cylinder diagram and checking the leads numerous times, i know the leads are on the correct cylinders!! why would the car now backfire?