Ok so I am from the UK and have just got an HTC Android phone. I have a data usage limit of 500 MB per month on my ty UK network.
I got the My Data Manager app to track my data use and at the top every time (of mobile data not wifi obviously) is 'Calendar Storage'. It uses up stupid amounts of data every day.
I have gone through all my accounts that are synced to my device (facebook,gmail,GMX,linkedin etc) and I have disabled the calendar sync for all of them that have the option available. Yet it still shows as using data every day on calendar storage.
Today, all of a sudden, the Calendar Storage used 93 MB of data! I called HTC, and they were very unhelpful. The best he could come up with was "a glitch in the program".
And I have The Weather Network, and it doesn't show usage as your theory suggests. I'd love to hear anyone else's thoughts or solutions