Why does everyone get different meals? Allergies? Taste?
Enlightened self-interest.
MIL (who is normal weight and has never had a problem with her weight) cooks old-fashioned Chinese home cooking, flavored with MSG and dripping with oil. When I was having fat-triggered gallbladder attacks, she'd make battered and deep-fried pork nuggets special for me, out of lean pork so I could eat it. "I make these for you! No fat!"
Another of her specialties is stir-fried lettuce (mostly prepackaged iceberg salad mix, but sometimes she'll do romaine). She refrigerates tomatoes and bananas, and will leave raw meat out on the kitchen counters from morning until night. She also doesn't believe in leftovers - if she makes it, it must be all eaten up. Oddly enough, she generally makes special food for herself (which she doesn't anticipate anyone else will eat, nor do we) at dinner for health reasons, but none of us can figure the logic behind those health reasons other than "my friend told me it was good to eat X."
Some of the stuff she makes is actually amazingly good. But if I eat much of what she makes 2 or 3 days running, she starts cooking for me on a regular basis, except she doesn't make foods that I'm willing to eat all the time, and then she gets grumpy that her food is being wasted and I get grumpy because I don't have the fridge stocked with stuff I like. We get along much, much better when I cook for me, and she doesn't. I eat from 9am to 5pm at work, come home and have a small portion of whatever I like best from what she made along with the rest of the family, and then get the rest of my calories (which is usually about 1/3 of my total) in between 7pm and 11pm.
My partner (who is overweight but not obese) cooks what MIL makes, because she'd rather not fight that battle. Her hours tend to be very different, because she needs a couple hours of peace and quiet every night so that she and MIL can both survive living in the same house. So she eats something she makes herself for breakfast around lunchtime, what MIL makes for dinner, and something that she makes herself for dinner around midnight. MIL is fiercely territorial of the kitchen, so taught none of her kids to cook, nor will she allow them to cook (even as adults) when she's present. She takes it as an insult to her cooking skills. So my partner has no cooking skills, and subsists mostly on frozen dinners and soup (either canned or ramen).
5yo kidlet is < 3rd percentile weight-for-height, and is the world's slowest eater, so we try to offer her calorie-dense foods. The food MIL makes is calorie-dense, but not particularly kid-friendly (too spicy and too salty, for the most part). She'll often have some of the main protein and some rice with butter, but get most of her nutrition from something plainer. She'll often have some of what I eat, or a 1-or-2-ingredient food that takes little or no preparation (a slice of bologna, or cheese on toast, or a piece of fruit). If she were taller (or I had the patience to listen to her drag chairs around), she'd get it for herself, but I'd just as soon have one fewer person leaving the fridge door open.