When a user selects a Project (say ProjX) and Issue Type (eg Task) in the "Create New Issue" wizard, then JIRA remembers these two values in the user's session as the
last selected Project and last selected Issue Type.
Next time the user creates a new issue, then these two values are used to initialise the selected values.
If the user browses a project or issue, then the selected Project is changed accordingly (eg to ProjY), while the "selected issue type" remains the same.
When the user now clicks "create new issue", JIRA initialises the Project to ProjY, and the issue type to Task, unless Task is not a valid issue type for that project.
In this case, it just defaults to the first issue type in the list.
This is inconsistent with other behaviour, and it would be better if the "default issue type" for the Project was used in this case.