Nhs surcharge ambiguity

I am on entrepreneur Visa and my extension is due in couple of months. As far as I know from the website it says that I have to pay the surcharge of 200 per year per person
Since I am applying for extension which is only for two years hence according to this rule I should be paying 400 only. But when I checked on the website which is given by border agency it calculates my surcharge to be 600 which is basically for initial application as that is for three years but for extension it should be 400

Can someone please clarify?

hi
my friend has recently applied for extension and he also paid for 3years which is 600£. once visa will be extended ukba would refund the xrtA 200 back (dont know the logic why but this is happening)

hope this helps

Thanks