This may be because the rows in your source categories are in the wrong order.
When you calculate a C2C formula at the sub category level, the formula starts at the first row in each of your source categories and effectively runs the formula a number of times, working on just one row each time.
So for instance, if you have built a multiplication using two source categories, the formula will multiply row 1 in the first source category by row 1 in the second. Then it will multiply row 2 by row 2, and so on until it gets to the last row in which there is data in both source categories, then it will add all the results together.
If you have 4 sub category rows in one source category, and only 3 in the other, then the 4th row will not be part of the formula.
Also, the formula looks at the rows as they were originally entered. Reordering visually using the up/down arrows does not change the order as far as the formula is concerned. You will need to manually re-enter data in different rows if you wish the formula to calculate differently.
This video gives a quick overview of Sub Category Level formulas: