Is it definitely saying 13.99 per month?
When you buy 90 or 180 days of ESO plus you pay the full cost all at once. So it lists the price per month (which is slightly cheaper than buying 30 days at a time) and then the price you will actually pay.
For 90 days it says "£7.99GBP/mo - 90 Day Membership - Charge me £23.97GBP every 90 days". Which means it will cost me £7.99 per month, but that has to be paid all at the same time so the total cost is £23.97. (The exact amounts will be different depending on which currency you're using.)
The other possibility is that you are being shown the price in two different currencies and the price difference is because of the exchange rate.