WorksheetFunction.Sum() Not Equal to Range().Value

My first guess is that it is a rounding error (http://www.cpearson.com/Excel/rounding.htm ) I notice that =(I4-sum(G4:G5)) yields something like 1E-13 -- not exactly 0. Anytime you deal with arithmetic in a computer (this is not limited to Excel), you have the potential for round off error, and your programming needs to be prepared to handle it.

Experienced programmers that I know never test for "exactly equal to" or "not equal to" in the way you are doing it here. They will account for this round off error in some way:

Is abs(a-b)<epsilon where epsilon is some small value that represents the necessary precision.
Is round(a,epsilon)<>round(b,epsilon) where epsilon, again, represents the level of precision that the test is required to pass.

I expect that is the issue you are up against, and that you will need to adapt your code to account for this round off error.