[gruez]

>That is incorrect, actually. I find it relatively easy to adhere to keto diet. But the moment I try to introduce more "diverse" foods, I gain weight.

Keto diets might be easier to stick to than calorie counting or whatever, but the fact that you bothered with a diet at all means you're selecting for people who care about their health.

[bluGill]

There is also a high chance you selected for people who were not eating well before. Or you selected for people that for genetic reasons are more likely to get fat.

Either way you cannot be sure your selection applies to other people.