Slushpool Miner Hashrate always Lower than Local Hashrate
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