This study says that women feel hunger more strongly than men due to physical differences in the brain. True or not, the idea that some people just feel more hunger for the same amount of calorie deficit could explain a lot.

Also, "calories in, calories out" is the second thought-terminating cliche[1] I've encountered today!

[0] https://www.bnl.gov/newsroom/news.php?a=110876 [1] https://en.wikipedia.org/wiki/Thought-terminating_clich%C3%A...