With files in folder you would stack all your 12 months vertically in one gib table. With lag column you could potentially compare your month values
If you use a loop with column append end node you have them stacked horizontally (similar to when you use joiner node for all your 12 files and one or more join criteria, e.g. customer name). Then you can calculate difference between the columns
as already pointed out by victor_palacios data sample is certainly helpful here.
Just trying to focus a bit more in the challenge (once I saw your example). We had a post in the forum happened last December month. The challenge was also focused in retail data and difference calculations.
There were some differences with your described challenge:
the requested granularity was based on a month by month basis (instead of a pinned reference month)
the target field was the product instead the client
the differentials were presented as quantities instead of percentages
However I think that there are few similarities with that challenge, and maybe you can get some insights from it, aiming to plan your next steps.
You can see a simple example here where I used your excel sheet to make sure my Quantity YOY matched yours with 3 nodes (I did something silly with reading in the data, but you can ignore that and just check out the 3 nodes Sorter, Lag Column, and Math Formula which will replicate what you did in Excel.)
The lag column gives you the lag specified. If you want a different lag from -1 then change the setting in the node. If you want to fix it to a base you can simply add this base number as a constant column and then divide by this column value.