The calendar preview is loaded into the email message from the recipient's primary account calendar when the message is opened. If the recipient maintains additional calendars in his or her profile, appointments on those secondary calendars won't show up in the meeting request calendar preview. If there are any changes made to the calendar and the meeting request is opened again, those changes are reflected in the calendar preview pane in the message.
Ideally, the calendar preview feature will save users some time because they won't have to open their calendar separately to view what activities they have booked for that day.