It might also be a case of paypal's currency conversion commission. They set it based on the currency your converting into. So 14.99 Pounds may get you 18.00 Euro with them, but if I had 18.00 Euros to convert into Pounds, they would only give me 13.92 Pounds. About 54 pence less than what my 18 Euros are actually worth. You get about 66 pence less than what your 14.99 is worth.
18.00 EUR = 14.4584 GBP & 14.99 GBP = 18.6628 EUR without commission I believe. I haven't even looked at these prices in a long time. I buy everything using dilithium/stipend ^^.