No you're not. You just haven't had the Virgin Media pricing confusion explained to you.
Let's look at your current pricing. You have a discount of £14. The discount is always apportioned as follows:
1. £1.50 towards TV XL, reducing the billed £26 to the quoted £24.50;
2. The rest of the discount, in your case £12.50 is apportioned to the BB. This reduces your 'price' for BB L to £13.50 which is what is quoted here:
http://shop.virginmedia.com/broadband/up-to-10mb.html
The £18.50 price for BB XL is quoted here:
http://shop.virginmedia.com/broadband/up-to-30mb.html
and it is a discounted price.
Your bill will increase by £5 per month after the three month period of no increase.
Hope this helps.
PS You will be billed £37 for BB XL but the discount will increase by £6 to £20 to compensate and give you the correct quoted price.