Without knowing exactly how Slushpool calculates your miner's hashrate, it would be hard to say for sure. But in general, mining pools can only guess what you did, and that will not be accurate. This is done by sending you blocks to solve (sharing) with an easier difficulty than what is needed to extract a block of real bitcoins, and see how fast you can solve it. Obviously, the speed at which you solve the blockage will vary so it will not be a precise way to measure your hashrate in the short term, and you will see a gap.
You have direct access to the machine, which should give you a reading of hashrate, and of course it will be different from the Slushpool estimate.