Cell reference adds another cell to its own value?

[justice-landes]

I'm using a VLOOKUP formula layered with other things to pull values from the Annual Detail sheet into the Invoice to Print sheet. For some reason, cell C15 on the Invoice displays the value it should have plus double the value that should be in C13. C13 displays properly, and so does the rest of the column it seems. The formulas are all consistent, so what's with this mystery cell (selected in the first screenshot)?

Invoice sheet showing the bad formula

It should be showing 4 from the selected cell here, but instead it shows 14 (and I checked, it still shows 14 even if the cell below it is empty). No matter what numbers I put in this sheet, C15 on the Invoice sheet shows F9 + (D9*2).

Overview sheet showing the source values for the invoice sheet

I'm on a Macbook Pro / macOs 11.6.3 Big Sur, Excel version 16.64 on 365 subscription. Here's a troubleshooting sheet where I tried to dissect the formula (the one for a working cell, C14, and the mysterious cell C15) and got no answers. Below that is the INDEX sheet, just because it's referenced in the formula so I figured I'd include it in case.

Troubleshooting sheet

Index sheet where both other sheets pull names and budgets from

[#2]

@justice-landes You provide a lot of information, but left out one important part. What is in the 6th column on the row headed with "YTD Spent"? The formula returns 24 as per the third screen-shot in C3, where you only show two amounts spent for July and August (4+10) for 14. So, either the formula summing the total spend is wrong or the sum range contains a one or more numbers adding up to 10 that shouldn't be there. Perhaps in hidden rows. Can you show a more complete picture of the sheet in the 2nd screen-shot. One that shows the YTD Spent row and the formula in column F on that row?