Most of these pools really don't know what your miner's true hash rate is. It can only guess the hash rate by looking at the number of submitted shares vs your difficulty. The actual hash rate of your rig doesn't mean squat if its not making valid shares.
That's not a fact, if you have such problems, you need to watch the total received rate "Hashrate" in %, and already from this percentage the number received of coins (Amount). If you get exactly what is stated on the website, you are losing half of their income, in his case it is 60%. I have sometimes a similar situation occurs, we have to restart the client (miner.exe) only after this, the stats on the pool begins to converge. I don't know why this happens, but it is almost always only one algorithm (lyra2v2).
That may be truer on the lower hash rate hardware. I'm only telling you what it looks like from larger asic based miners (x11, script for me). Sometimes it reports high, sometimes low... and it's based off the shares the site is seeing. Nicehash actually had a page dedicated to explaining this. Here's where someone posted it to reddit... last couple paragraphs.