Belgian Post Office

Could someone please explain why the Belgian Post Office seems so particular when posting Birthday cards, Christmas cards or even Saint Patrick's day cards :)It appears to me there is a "very strict" interpretation as to the what constitutes a standard size letter. What appears to be a formal business letter size, However anything else and they hit you with charges. I just went to post a birthday card (19.5cm x 14cm)to the USA and it cost €3.87?
Most birthday/greeting cards dont conform to formal business letter size.. the price just seemed abit high to me? It seems odd that in other european countries I've lived in they had one price for the stamp and aslong as it was with in reason and just the card it was fine. TIA