To make it easy for you to add reservations to any calendar, we send an .ics (internet calendar standard) file along with your confirmation emails for your reservations. This is why you may see an invite at the top of the email to "Accept" or "Decline"
If your Google calendar is set to automatically add invitations to your calendar then your reservations will always be displayed on your calendar.
If you prefer not to have these added to your calendar by default, you can change this Google setting. From your Google calendar, click the settings icon and choose Settings.
Toward the bottom of the settings page, find the section titled Automatically add invitations to my calendar, like the screen shot below. Choose "No, only show invitations to which I have responded."