The default growth rates and other settings, found on the Preferences screen in the Plan Preferences (to the right), are used as the initial defaults for the plan.
You can, however, easily override these defaults by making edits at the item level - e.g. changing the inflation rate for a particular expense on the Expense > Basics screen, Advanced Settings, Inflation panel.
Any edits made to settings at the item level will override the initial defaults from Plan Preferences. Moreover, once edited at the item level, these settings will remain as edited for the item, regardless of any later changes made in Preferences.
How to revert a setting to the plan's default
If you need to revert a setting that was edited at the item level back to the plan's default, simply clear the value -- e.g. delete the Inflation rate from the Expense screen's Inflation panel -- and click Update. The item will once again use the default value taken from Plan Preferences.
Note: If you are viewing a scenario, clearing the value will revert the setting back to the value from the base plan. To ensure that this value is taken from Plan Preferences, also be sure to clear the value while viewing the Base Plan.
System Preferences vs. Plan Preferences
Preferences - A general introduction to system assumptions and calculation settings