Because it's where you're getting your calories, not just getting them. I used to calorie count on MFP as well when I tried to get to target after DD1. I ended up eating rubbish to stick to 1200 cals, just to get that last half stone off, but when I actually did SW and ate more, I got there. I lose more when I eat more. Sounds daft but it's true. It's not just calories, it's fibre, water, nutrients etc.
My assumption is that when it works it is heavily reliant on 1/3 superfree.
Now I have around a stone to go I'm having to adjust SW official line a bit otherwise I can't lose weight. I'm inclined t think It works really well to begin with when you have lots to lose because, lets face it, you have to be eating a lot more than you need to put on so much in the first place ( 3. stone of crisps and petrol station sweeties did it for me) . Now I'm a top end size 10 I need less fuel, so portion sizes have had to come down a bit, superfree be very generous and I reach for raspberries before a banana.
You are eating too much, you had 2 whole tins of beans in one day, you are obviously not stopping when you are satisfied,the whole idea of SW is to fill up on superfree foods and you still have to watch your portion sizes. Get a portion plate and divide your meals up properly you will then see how you are still overeating, you should have had a quater of the melon, half a tin of beans on toast with a bowl of salad and peppers etc for lunch and then some chicken, rice and peas for tea, more substance but less calories and also plenty of super free, when they say eat as much as you like what they really mean is eat as much free food as you NEED. very diffrent things to a compulsive eater, you need to control yourself.
I think it does depend a bit on what you were eating before the plan, but I lost 3 stone in 3 months doing SW and eating ridiculous amounts of food, although that was on the 'old' original and green days. I've also just rejoined and have lost 8 1/2lb in two weeks. My days food is something like:
Breakfast: 2 large bananas, big dollop of 0% fat Greek yoghurt and 35g of muesli or a 4 egg omelette with mushrooms, tomatoes and HE of cheese.
Lunch: big bowl (or two) of soup, HE of bread, fruit and/or veg and/or mullerlight.
Dinner: huge salad with lean meat/fish and new potatoes, spaghetti bolognese, roast etc. although I usually fill at least half my plate with superfree at dinner.
Snacks are hiding bars, fruit, cold meats or 'free' yoghurts and I admit I don't have many 'syns' unless they are part of a meal, although I occasionally have them.
I do eat to hunger, but sometimes that means I eat a lot. Last night I had a ridiculously huge plate of pasta and was still hungry after, but I think it balances over the week as other days I'm not so hungry.
I do think you'd benefit from more superfree though.