In SharePoint 2010, the calendar views are rendered via a script on the client side. This is a significant improvement over the heavy calendars of the previous versions (2003 and 2007), and in the 2013 version client side rendering has been extended to all list views.
Unfortunately, the SP 2010 calendar also came with a bug that I reported a couple years ago. It is made of two layers – one for the grid and one for the events – and when the calendar gets resized the two layers get out of sync. Really annoying! As far as I know, Microsoft has never fixed this. I keep receiving reports of this bug from users, especially as it occurs when calendar views are combined with the Easy Tabs.
If you have to deal with this annoying behavior, here is a simple workaround: go to the Web Part settings and give the Web Part a fixed width.
Hope this helps!