As an Admin / HR user you may find that some events appear on the Calendar but not in your 'Upcoming events'. For example you may see Mandatory Holidays relating to other Sites in your Calendar but not in your Upcoming events, This is because Upcoming events only displays events that apply to you as a User. The Calendar on the other hand displays ALL events, regardless of whether they apply specifically to you or not.
Take the HR user below for example, Aimee-Louise is based at the London, UK site and therefore has the UK Bank Holidays displayed on her upcoming events as they apply to her:
However when she views the calendar she can also see the Bank Holiday 'Orthodox Easter' on the same date. This Bank Holiday is not relevant to her as an Employee but does apply to the Paris, France site.