Jonathan Bailor has a nice explanation for this, where he uses a clogged sink analogy. Every body has a rate of flow from the faucet (food intake) to the drain (metabolization and elimination). If you clog the drain and make it inefficient, the sink fills up to a higher set point (you gain weight). He says that it's a hormonal issue, and it may or may not be genetically established. His key to fighting obesity is for people to switch to high quality food that unclogs the hormonal sink and establishes a new, lower set point. Basically that's what keto and LC eating does, but Bailor's approach gives a little more dietary latitude.

(It's all interesting stuff backed by actual scientific research, which is great. Like I said before, the problem for me starts when Bailor starts to hawk his program, his books, his supplements, and so forth. I guess he has to make a living, but somehow the information loses some value when I see it as advertising.)