The difference between the expected and actual performance represents a variance that is considered when analyzing solution performance. Root-cause analysis may be necessary to determine the underlying causes of significant variances within a solution. 

One of the key aspects of this task is for the business analyst to understand the stakeholder's expectations in terms of the desired business value they seek and then compare this with the actual performance measure results. If there is a large variance between what the actual business value is that the solution is delivering and the desired business value, then it is the role of the business analyst to work out why there is a variance. 

Let's consider a practical example:

The stakeholders of an enterprise had an expectation that the transaction processing times will increase by 20% when the network data transmission capacity is increased. This business value did not come about when analyzing the actual performance measurement results. It is therefore now the role of the business analyst to analyze the results and other factors to try to understand the variance in results. Some of the other factors that could be analyzed in this scenario might be the size of the server, the number of applications or processes running on the server, or perhaps even the age (and efficiency) of the surrounding hardware that is supporting this change.

It is a good practice to consider the holistic solution and the relevant business processes, goals, and other key relevant factors when diagnosing the root cause of such variances.