A hit prediction for Peru on 15th and some questions to seismologists

 Here is one more hit prediction

An Earthquake of 6.1 magnitude struck Peru on 15th June 25 as against predicted date 16th June ( with + or - one day margin)
  Details
1) Magnitude..6.1 ( 5.7 to 6.1 as per different institutes)
2) Time .16.35 UTC ( Universal Time)
3) Epicenter..12 S , 77 W

Not only this ,the epicentre of quake follow my epicenter location hypothesis
( for details see
https://earthquake-prediction.blogspot.com)
The quake struch when Moon was setting at epicenter ( 30 minutes margin)

I have one question to seismologist
This quake is noted of magnitude varying from 5.7 ( USGS) to 6.1 by local seismometers.
Is it not possible to measure quake magnitude accurately ? Why every Institute have different magnitude?
Are we conversent enough with Earthquake measuring methods, and why do we claim so much about plate tectonics, when we can not measure the quake occurring on ground?