Maybe it's pre-tax but not inclusive of rebates or some weird thing?
According to the stats, your actual profits were $18,154,908, not the ~$15.5M that you say. So that's the difference.
It's possible also that the figure you see on your Income Sheet when you do Quarterly, wouldn't be the same as what the stats show, I'm not sure. The Income Statement figures are all by Week, making some months 4 full weeks and others 5 full weeks, and quarters are the same way -- a week is counted in a quarter as long as it starts within that quarter. If the Stats screen is counting true quarters, but the Income Statement screen is counting full weeks that begin in a quarter, you could have a different of 1 weeks worth of profit on one screen but not the other.