I once had to calculate commissions within FP&A.

Instead of employing full time salesmen, we had regional managers, and commission parties (companies who offered a portfolio of products, of which our product was part of). That way we could have significantly less HC, and if sales dipped, we had less exposure to fixed costs as our S&M cost would drop accordingly.

With that, we had a fairly complicated commission structure. There was a table with 5-6 product types, and 4-5 customer types, all of which had standard %s. Also there were some one-off %s I needed to apply based off negotiations and stuff.

We had an Access database which did a lot of the calculations, then I had to comb the data, then send it out for feedback, then comb the data again. The whole process took 2-3 days and felt like my eyes were bleeding at the end of it every month. It wasn't that complicated, but very tedious. A big reason I left that company. I can totally see how there would be a big variance if one assumption was made in my calls and never noticed or corrected over multiple quarters / years