Slushpool Miner Hashrate always Lower than Local Hashrate A Short Story

There is an Antminer server farm I provide support for with over 100 BC servers—mainly S9 and T9 models. There are three specific S9 miners that are side by side right next to each other on the rack which continually show lower than normal hash rate in Slushpool. 

When I check these servers in Slushpool they all three fluctuate often ranging from 11-13 TH/s but the web interface and command line RT and AVG hash rates are always normal with no fluctuation. Every time I check Slushpool though, it's almost always lower than normal and I've checked up to nearly a hundred times in one day just to ensure I was being thorough.

Question

Is there anything common or something someone else determined that could cause a miner hash rate to always show lower in Slushpool than what it shows from the local server? 


  Notes
  
  
  I've checked both command line and web interface on all these and see the same thing; normal realtime and normal average always
  every time I check but with Slushpool, it almost always shows it is
  hashing lower by 1 or 2 TH/s.
  Typically when other miners in this farm have a lower than normal Slushpool  hash rate, the local servers have a value that correlates much closer to that hash rate so I'm
  trying to understand if there's something I don't understand here
  which may explain this.
I believe that whatever is showing up on your dashboard is probably the most accurate picture of hashrate that you're contributing. Because you're mining to a pool, there will inherently be some, slight mismatch.

I wouldn't consider fluctuations a major issue unless it says you're producing far below the advertised hashrate. 

I do know that Slushpool uses a "scoring hash rate," and if you want to know how that works in some more depth, here's a link to an article that Slushpool released. Hope this helps!

https://blog.slushpool.com/effective-hash-rate-af9c90529f6d