Ashful - Nice catch!  Thank you.  Thought I knew what I was doing, but I didn't.  Back in the day, it seemed that mfg's spec manuals specified some lubricant before torquing.  Then they listed the torque values for each fastener.  I just extended that to spark plugs.  
I just checked and found information from NGK where they specifically state NOT to use any sort of anti-seize before torquing a spark plug.  They warn that doing so may result in over-tightening and damage to the plug and/or cylinder head!   They also said that all NGK plugs are treated with corrosion resistant plating, so 
do not require any corrosion protection. 
I've been using anti-seize on spark plugs for probably 40 years and have not had a problem, but I'll never use it again.  I always torque to the low value in the range and perhaps that's what's been saving me.  There's just something about a steel thread spark plug screwing into an aluminum head that has always made me cautious.