I really shake my head when someone posts "The sales person said this plan would be all I need".
There is no way anyone can accurately account for a users browser settings, browser extensions, browsing habits and a multitude of system settings in a brief phone call.
Websites change, usually without warning. Recently, using IE, (no ad block or browser extensions) I accessed some pages of election results from MSN.com. Most of them contained auto-start video somewhere within the page ... that you didn't see right away and therefore it consumed data.
I keep IE "clean" so as to have a source of browser comparison if needed, and yes I could have switched to another browser and avoided that extra usage.
There is no way anyone could give more than general ball park figure. If a user leaves an auto refresh tab open it is going to use data like mad.
That really is more of a case of a user not understanding how their computer and a constantly changing internet operate.
Its a user problem, not an ISP problem.