This is the answer. I used to wonder why long rides would be over by 1-2000 calories when using the same GPS file with heart rate data on certain websites/apps - the answer is that some apps add the calories you’d be burning anyway, so if you exercise for 12 hours and your BMR for a day is 2000, it’ll add 1000 on to the ‘calories burned’.
TR will give you the number of calories that you burned on the bike, accurate to one of the highest levels possible (as cycling is an activity that’s relatively easy to measure output for)