I can't post comment yet, so I will post it as answer.
Calories estimates are for reference only and that is nowhere accurate at all.
Your body consumes calories even at rest, our body requires calories to operate at all time(heart-beat, respiration, etc.). I am going to say what all people say about weight losing strategy. Obviously the application you are using is misleading and the most effective strategy to pay attention to what you are going to eat.
"Do sports and do not eat more than you need".
As a postman, I can imagine you walk with a heavy bag every weekday, but the main factor in consuming your own calories is the intensity of your activity, you can walk slowly or in normal speed for 1 hour, the calories burnt/used are nowhere near race-walking for 30 minutes.
I mainly eat well with the occasional boost of the naughty stuff
This could be a problem, if you boost yourself with naughty stuff in short interval between diet and assuming that naughty stuff is a low quality calories food, you are not going to lose weight. Your body will store extra calories when you have more than you need, low quality calories provides high calories and does not fill your stomach well enough. If you are hungry, try something else.
If you wish to lose weight, then pay attention to nutrition label, not only the calories, but the other nutrition like fat and sugar. Usually you won't find nutrition label in restaurant, but you should be able to determine what is good and bad for your nutrition goal. You will be more experienced as you try, I managed to go from 40" belly to 34" in a year by myself using the same strategies I typed. Resistance training can help too, but for this topic, you look for information in relationship between nutrition and resistance training.