Why are there so many different patterns of weight gain in pregnant women?

Genetics. Every woman is different so every pattern of weight gain (and every pregnancy) will be different. As a general rule of thumb, a normal weight woman should gain between 25-35 pounds while pregnant. This often comes as a spurt in the middle of the pregnancy. Overweight women should gain less and underweight women more. Good nutrition and exercise are recommended for all pregnant women.
Emotions and hunger. Pregnancy does increase hunger cravings. The body is simply wired that way. Emotions can play a part for sure. Depression an stress ten to trigger comfort foods. Usually harder to exercise. Always address your eating patterns with your OB doctor.