Hi!
My take on this is that despite the question involving two parts - the why and the what - the idea of the profitability structure is that it involves determining the drivers for the why. So it should be all encompassing to cover the why and the what.
The structure I recommend using is one that involves a horizontol and vertical approach (essentially a matrix).
At the top (horizontally), I would draw out the typical profitability formula. From left to right I would write revenue = P X Q and cost = unit cost X Q.
Vertically, I would detail the various ways those drivers affect proftiability:
For P - price change ; and new price
For Q - existing volume change; and new volume change
For both P & Q - mix of product change (you could sell a different mix of your own product that delivers different levels of profitability...think average price)
For unit cost - you have the effect of fixed and variable costs
For Q (from a cost perspective) - you have inventory management
Now that you have detailed the structure and matrix quite comprehensively (MECE). You can then use this to allocate the reasons why. I can prove to you that this covers all aspects.
Possible reasons for Revenue change:
1. Competition (old or new)
2. Consumer preference change
3. Other market share drivers (sales and marketing efforts, customer relations, new customers etc...)
4. M&A activitiy
5. Increased market size - whether it is natural growth, new method of use for products, non-customers becoming customers etc...
6. product mix
Reasons for cost changes
1. economies of scale
2. fixed costs upgrades
3. poor forecasting
4. product mix
5. sourcing changes (better or worse vendor, vendor bankrupt, vendor price change, shortages etc...)
6. supply chain or other forms of integration (vertical or horizontal)
7. logistics (change in geography of customers, outsource vs. insource, batches shipped at a time etc...)
etc...
Each of those reasons could be traced to one of the structure drivers or levers. Whether factors are internally driven or externally driver, the why could be placed into one of the matrices.
DM me for more information and on how to identify why reasons and levers are more likely than not the case. I can help with practicing the model and detialing it out.
Best,
Rakan Bilbeisi