The answer might be surprising - magnetism.
We’ve all played with magnets - sticking them to metallic objects, picking up metallic objects with them, and such. In doing so, we noticed that while magnets will stick to some metals (e.g. iron, nickel) while not sticking to others (e.g. copper, aluminum). The human body requires some metals, in trace amounts, for proper function (e.g. zinc, chromium). While these metals are found naturally in fruits and vegetables, some foods are fortified with them. When digested, any excess metals are eliminated. Some metals are used by the body while any excess ones are eliminated. During this process, minute amounts of metals are mixed in with other gases that form in our stomach and bowels (e.g. methane). The higher the metal content in the foods we consume, the higher the amount of metal in our bowels. For example, many dark green leafy vegetables are high in iron (e.g. spinach, kale). Accordingly, the more of these we consume, the more iron is in our system.
Scientists discovered long ago that the earth’s North Pole emits a stronger magnetic pull than its Southern Pole. This means that magnetic metals will be drawn to the north than the south (the small the amount of metal the stronger the draw). Therefore, the closer one is to the northern pole, the more bodily gasses will be expelled though their mouth than their anus, while the opposite is true as one nears the southern pole. Everyone knows that north is up and south is down (stated on every map). Consequently, when one sits, they are closer to the southern pole than when they’re standing. This is why sitting induces more flatulence than belches (and vise versa).
Element 99 