It isn't the same thing as making your body think that it is already pregnant: your body does not start acting like it would during pregnancy. It is, however a handy explanation for most people, and much easier than explaining how estrogen, progesterone, LSH, and FSH work during the menstrual cycle and how adding in some estrogen and progesterone inhibit ovulation.

Statistically, you are no more likely to gain weight on hormonal contraception as you are to lose weight or experience no change in weight. Doesn't mean that your weight as an individual will change while taking HBC. For many women, the initiation of HBC happens to take place at the same time, roughly, as expected changes which mean weight gain (late adolescence, for example).