Multiple years of Financial forecasts: differences?

I have a table that contains a bunch of financial forecasts for multiple years. For example purposes, I simplified it to it's bare essence:

Note for example that:

• In the forecast of 2016, for the account number "61", we predicted a budget of 1.200,- for Year+1 (so for the year 2017)
• In the forecast of 2017, for that same account number "61", we predict that same 1.200,- for the budget of that year (so for the year 2017)
• So in this case, the "forecast_of_2016" predicts the same amount for 2017 as the "forecast_of_2017" predicts.

What I would like to do, is find the differences between forecasts. For example:

Note how:

• In the "forecast_of_2016", for the account number "61", we predicted a budget of 1.400,- for year+2 (so for the year 2018)
• In the "forecast_of_2017", for the same account number "61", we predicted a budget of 1.500,- for the year+1 (so for the year 2018)
• So in this case, the "forecast_of_2016" predicts a different amount for 2018 as the "forecast_of_2018". I'd like to make a report that shows these differences. I'm looking for some help to get the model right.

Maybe an important remark:

• Note how some account numbers are in the "forecast_of_2016" (e.g. "62"), but not in the "forecast_of_2017". That should also mark as a difference
• Similar: some account numbers are in the "forecast_of_2017" (e.g. "63"), but not in the  "forecast_of_2016". This too should also be mark as a difference

I'd appreciate if you could help me with this. Thanks for reading.

Kind regards,

Andy