Oh the faux bafflement and ridiculous scenarios people are inventing here to bully the OP are astonishing.
It's pretty fucking simple... wait staff bring out food, hazard a guess at who to put which plate in front of.
If two people have ordered the same meal but one appears larger than the other (or insert some other difference) they make a judgement.
What all this shit about chefs knowing who ordered what, or people taking food off one plate and onto another is fuckery of the highest order.
Yes, I've had a waiter make a judgement based on my appearance as to which meal was mine on many occasions.
Im a fat woman, my sister is variously a normal sized, slim or even very slim woman.
Almost. Every. Single. Time. I will be given the larger meal, the one with chips rather than salad, the one with the massive burger rather than the healthier option.
On most occasions, they're wrong, my sister can eat like some sort of human dustbin and I have gastropareisis and often struggle to eat a bloody starter.
I really couldn't give a shit and we tell them they're wrong or just swap over if they've been too quick to put them down.
I can easily see them deciding things like the woman has the alcohol free drink, the wine rather than the beer, the smaller portion, the salad rather than the burger etc etc.
I can't get worked up about it, I wouldn't start a thread about it - they've gotta offer the plate to someone and they guess based on frequent experience.
But to pretend this is fictional purely because it has never happened to you is also fucking ridiculous.