Check integrity between servers
First of all your replication is not synchronized. That means there can be always slight differences between master and slave. if you change configuration like below your master won't return commit until the replication processes. 


  synchronous_standby_names = '*'


In your case, synchronized replication can be problematic because your slave is quite distant from the master so that the commits get slower.

Still with synchronized commit running a query 2 times in different transactions can get different results because of data changes. 

I recommend you to read the blog; Evolution of Fault Tolerance in PostgreSQL: Synchronous Commit.