Why is my event being added to the wrong calendar service?

When users click an add-to-calendar button or link to add an event to their calendar, they are able to choose a calendar service to add the event to. Sometimes when users select a specific option (for example Apple) the event will be saved in a different calendar (for example Google). This is because the user has selected a specific calendar provider (in this example, Google) as the default calendar for their device. All .ics files (events and calendars) will typically be opened by the calendar app that has been set as the default in the user's system, even if the user selects a different calendar service.