It is bowed because the opportunity cost of butter is calculated based on the number of guns and butter the economy is producing. If the economy produces lot of butter, people who are best suited/skilled to make guns are engaged to produce butter. As a result decrease in each unit of gun production results in a small increase in the butter production. The opportunity cost of producing butter is more and the slope is steep for the PPF. While on the other hand, if the economy is producing lot of guns, people skilled in butter production are being used to produce guns. So for each unit decrease in gun production results in huge increase of butter production. The opportunity cost of producing butter is less which means the slope is less resulting in a flat PPF.