srinika
It's been a long time since I wrote exam but what I hear is:
There are numerous versions of the exam to make copying difficult.
Therefore it is possible that the toughness of the exam's would change also (luck of the draw I guess)
As far as the Gov. standards, Yes there is a "minimum".
I am sure they base the exam time limit on a per/question time allocation, therefore some exam's my be slightly shorter.
Just a guess