I'm not sure where that particularly statistic is (I wasn't the one who originally posted about that) - but they were just articles I've seen that explain it too :)
The NHS can only give things on average, but, everybody is different,
With calorie intake - if you were to see a nutritionist and get your own diet plan, it would differ to every other persons diet plan.
So 2000 is the recommended amount 'on average' for an adult woman to maintain their weight, (this is also going off the basis that this average person also exercise the average amount) but there are other factors that may tweak this amount, such as age, etc.
So as some examples
Say you have a 20 stone woman, who isn't very active, but on average she eats 3000 calories a day. & she wants to lose weight, starting to have 2000 calories a day instead may be what is needed to shift some weight to start with, 2000 is what another woman may need to maintain, but this lady is used to 3000, so 2000 for her is considerably less.
But on the other hand of it, say you have a lady who is 5 stone, and needs to gain weight, she's very active and isn't much of an eater, she usually has 1000 calories a day. To gain weight, she may only need to lower her activity levels and up her calories to 1500 to start with.
Then each persons height will play a part too. A 6ft woman will need more calories than a 5ft woman to maintain a normal weight. Etc
There's such a great grey area with it all,
2000 calories is the amount for an average, moderately active female adult.
You can lose/gain weight by altering your calorie intake and not exercising, you can also lose/gain weight by altering your exercise routine but not changing your diet - it's usually most effective to do both, but it all differs.
The best thing to lose weight, would be to work out your own calorie intake & activity levels prior to starting a change in routine, then you can try and lower your calorie intake slowly hand in hand with upping your activity levels.
There's also the risks involved with suddenly changing your eating/exercise habits.
If an obese person is used to no activity, and eating 3000 calories a day - it would be very risky and unhealthy for them to cut their calories down to 1000 and suddenly start intense exercise.
(I hope I make sense lol, but the NHS website and others make weight loss seem so simple, but it isn't, and everybody is so so different, we should treat things like this as a personal journey, we can't all do the same things)