Unfortunately I may not be able to fully answer because I don't know your data set. But I would guess it has to do with your rounding here:
ROUND(SUM([Sales] )*[Fulfillment Rate],0)
For example, if you added 4.4 and 4.4, you'd get 8.8, which rounds to 9. But if you rounded them before adding, you'd get 8. Tableau's Grand Totals run on the overall value - so it will round the sum of all sales, rather than adding up the individual rounded values. So the total you're seeing is likely correct, just the rounding is causing it to look funny.
Instead of rounding in the equation, consider just using SUM([Sales]) as is and formatting the data to show 0 decimal places.
Let me know if this doesn't do what you're looking for!