The default calendar in Outlook is the calendar folder which is placed within your default data file mailbox. This doesn't consistently have to be same as your default mail account. When a new meeting request is receive, Outlook will purpose that calendar to establish item even when you received invite message via distant account.
To arbitrate which mailbox data file is currently set as default within Outlook, purpose Account setting data file tab.
Outlook 2007 and previous
Outlook 2010 and 2013
To change default data file, elect it and press "Set as Default" button. If you have elected a data file which can be set as default, Outlook will now apprise you to reopen Outlook.
Note: If you are not assured which data file calendar endure in, switch to folder list navigation (CTRL + 6).
When you added an Exchange account to a mail profile, the mailbox data file will be set as default and can't be replaced. When you have n number of exchange accounts configured in Ms Outlook or added supplementary exchange mailboxes, then respective mailbox or account will use its own calendar folder as the default calendar. This will block it from getting mixed up.
Analogous to Exchange account, Hotmail/Outlook.com accounts will purpose their own calendar folder as default for deal with anything related to calendar.
IMAP and POP3 accounts will use the calendar folder which consists in default mailbox data file as default calendar. This is also case alike when you have authorized POP3 account data file of its own and created calendar folder in there.