@Mabby Plain luck, I guess. On the Formulas ribbon, I selected "Show formulas", so that I could see what you really have in all the cells. I noticed quite a few inconsistent formulae, but the one in T269 stood out. There's not really much more to it.
With respect to inconsistent formula, you may want to consider looking over columns E and F in particular. Quite a lot happening there which is bound to create problems. The screenshot below shows the content of just a small area. Notice that column E has formulae and constants and column F mostly has SUM formulae, and then suddenly just a reference to one cell. Better to devise a rule that allows you to calculate GST with a consistent formula (if possible) and use consistent formulae in columns that sum two other columns
With regard to the formulae themselves, =PRODUCT(D249,1/11) would be easier to read (in my opinion at least) if you would use =D249/11. Similarly, =SUM(D249,-E249) could simply be written like =D249-E249.
