Dutchsinse said USGS often, if not all the time, downgrades the quakes over M6.0 to M5.9 so that people don’t panic and feel safter.
This is just like the price in a store. You feel cheaper if the price is $5.9 than $6.0.
In the conclusion, we cannot trust the data of USGS because of their manipulation of the real data. USGS is the US Government agency to manipulate all the data they collect.