I am not well versed in airline yield management systems, but will give you a view that might explain the logic behind the results, from a product management perspective.
If your business has historical records and an algorithm that can interpret these, it might find that
Product A has a constant value
Product B has a constant value
Product A + B combined has a variable value, depending on seasonal or other events, i.e. specific demand driven circumstances, which can be data mined and exploited.
Thus, applying for A+B might give a variable price, whereas the separate components may have constant values.
This is speculation for airlines, but I know it works in a very similar way in some other industries.