Profitability calculators like whattomine.com can only show you what the current profitability is based on if you were to sell coins you already mined at that very moment. They don't and can't take in to account other variables like price fluctuations, difficulty fluctuations, pool round variance and different pool/exchange fees. Unless you have a farm where you can mine enough coins to cash out in time to exchange them for the coin you want quickly, the profitability you see will not be accurate.
So, i thought about this and I did a test. I mined SIGT on zpool.ca for a few days. I checked SIGT pricing and it never dipped below about $8.00 a day on whattomine.com. Each day I checked my balance and I got about $6.20 each day after exchange. I did this experiment on NiceHash and I averaged about $8.00 day, what I was expecting. Does this make sense?
Again, whattomine doesn't take in to account other variables that can and do affect your profitability. Pool round variance, rejected shares, pool/exchange fees, Tx fees, etc. all affect your bottom line by as much as 20-30%. By default whattomine also uses average 24H difficulty and profitability in it's calculations instead of real time. The best way to figure out what your best profitability for your rig is, is to try different pools if one isn't working for you.