What make/model of miner?
How long has it been running?
Does it use modified firmware or Braiins?

Or, are you overriding the pools difficulty setting?

Well most miners take an hour to actually get an average close to what they are really doing.
If you are running any 3rd party firmware, and the pool isn't blocking it, they are also taking a % of your hash rate.
The workers page on the pool gives lots of details about each miner and the invalids,
and Help->Stats explains a lot of that page if you are logged in.