Surprise car tax increase

Sorting out the tax disc for some of the vehicles; hopped online to the direct.gov, entered the details and... wait that's more expensive then the price listed on the form.

Now in nice red letters is does state on the form that - "The rates of duty were correct at the time of printing. A different rate may apply following any budget statement". Except the budget statement was the 23rd March twenty-three days ago and we received this in the post yesterday.

So either the DVLA printed this off prior to the 23rd of last month then hung on to it until about Tuesday/Wednesday or they haven't been able to change the figures in the month they've had available to them.

Do anyone think that a business could get away with, say, not applying the VAT change on their invoices for a month and then telling the customer they owe more than stated?