I'm reading about digestion today and it's suggested that food stratifies in your stomach (fats and lipids to the top, smaller proteins and carbs in the middle, and heavier proteins at the bottom). The evolutionary explanation this book gives is that we process more readily available food sources first. So polysaccharides get squirted into the duodenum for digestion and absorption for use ASAP and then we wait on proteins and fats. Is that accurate though? Fats are digested slower than carbohydrates but is that due to something else? (Availability or large numbers of enzymes for this process?) More research required!
I always figured the mix in your stomach was constantly moving as it gets churned with acid and enzymes. I guess fatty acids would float in an acidic environment. I do recall a factoid from a class that floating poop can be due to unprocessed fats (side effect of things like those weird Pringles) that make it through your system too fast.