You've misread it
If you don't have an inclusive allowance there is a charging scheme where once you get to 25MB in a day you get charged an additional cost if you go over
http://www.virginmobile.com/vm/gener...hop.sm287#Data
http://www.virginmobile.com/vm/gener...ges.shop.sm142
But on most contract tariffs the allowance is monthly e.g 500MB or 1GB