Why do so many people say they can't reach their calorie goal?

There are a few different kinds of posts. There is the "oh, oops, I ate all my planned meals and have only 950 calories, should I eat more?" ones, or the related "I am not hungry, do I need to eat more?" ones. This basically happened to me -- I aggressively cut down on carbs and out all extra fat (cheese, added oil) and was choosing lean sources of protein, and when I started logging (after doing it for a little while) was surprised at how low my calories were. I did have an initial thought of "so long as I'm fine, isn't low cal better?" before coming to my senses and adding back in some fat (and increasing portions of other things).

I think related to this is that some have been eating quite unfilling diets so are surprised that they feel fine on low calories, and I also think many assume they will be hungry so interpret the fact they are not as "I should not eat." There's also lots of confusion about how hunger works (and fear of eating when not hungry or eating "bad foods" -- yes, this is a problem often), and a culturally/dieting industry sense that "the lower the better" or that the faster the loss the better (I blame Biggest Loser, in part). Also, often there's a self-punishment or machismo going on -- you've told yourself before starting it would be awful but you would be strong and can take it, losing the weight (sometimes "losing the weight fast") is worth any sacrifice, so there's a resistance to being told it can be easy, fun, and involve some treats, not a super puritanical* disciplined diet.

All this I find quite easy to understand.

But there are also posts where people claim to be SO STUFFED that they cannot manage to eat more than 900 or whatever. Often these people aren't even eating diets that look particularly filling to me (I remember one example that had barely any protein or veg, lots of bread, tiny portions). Usually I read these as ED-related in some way (although I don't say that, since it's wrong to diagnose online) or asking for permission to eat way too little or basically fake -- some kind of performance art or show-offy "look at how little my appetite is!"

However, if the claims in these are true -- if the person really is eating so little and feels physically or mentally incapable of eating more -- I think this really needs a trip to the doctor. It could be a medical problem, it could be thinking they really should be stuffed on so little or (in a few cases) fear to eat "bad foods" that reaches the level of feeling unable to eat them, or it could be that they've eliminated everything they like and feel stuffed because they are trying to force themselves to eat foods they dislike (or prepared in ways they dislike). A good dietitian could help with all of this, as well as a therapist if it's at the level of an ED, but it's all a medical issue, IMO.

*An aside: I watched that show where Sue from British BakeOff and some food critic pretend to eat like they are in different historical periods and in the Restoration one the remaining Puritans seemed to be the only ones who ate a halfway decent diet here, so I want to say I know I'm unfairly slamming Puritans in using "puritanical" in this manner, but I think it fits!