Microsoft in their infinite wisdom have two defaults for reminders on meeting requests, one is for appointments and one for all day events. The option in Tools -> Options is for appointments/meetings only and the defaults for all day events is hardcoded to 18 hours and can’t be changed. Now depending on how you create the appointment and which view you are in will depend on which default you get.
If you are in month view and click on a date it will use the all day default (hardcoded to 18 hours) even if it is not an all day event! If you click the new button or are in week or day view it will use the meeting/appointment default (whatever you set in the options).