Why isn't my diary description changing when I edit the job description?

When you add a new event it takes the description from the job itself. This is where confusion may be occurring. 

The diary description and job description are actually two separate things. When you edit the job description it will not change the diary event description (and vice versa).

This has been done because you can have multiple diary events for a job. Therefore you may want to have different descriptions for these events.