This is very strange.
I use Google Calendar Sync to sync an Outlook/Exchange calendar to Google Calendar.
On that machine I authenticate with two Google accounts, one has two factor authentication.
It looks like Google Calendar Sync is posting transactions to both accounts even though it has only one account's password.
At least, that's the best explanation I have so far.
Again - one of the weirdest things I've seen in a while, even allowing that sync is fraught with weird errors.