Why we can not measure the quake accurately even after it has occurred?

  I wander how can this happen ,with modern methods of measuring the magnitude of an Earthquake?

The quake of 3rd November, at Nepal is reported as 

6.4 by NEC (National Earthquake center), 

5.7 by USGS

and 5.6 by European units

And beauty of all this ,I was asked ,by several seismologist to predict quake accurately.Even my + or - 0.3 magnitude is not enough.

  When we can not measure the quake after it has already occured ,how can you ask someone to predict accurately, with no margin?

  Think Logical

   Think Out of the Box