The TrueBike calculates your speed based on your power. It takes into account your rolling resistance (your weight), air resistance (your height and weight), and climbing resistance (your gradient and weight). External apps also make these calculations, but the assumptions in the programs might differ slightly. This can cause small differences.
Larger differences occur when you ride in groups, which significantly reduces your air resistance in those programs compared to the resistance assumed by your TrueBike or TrueTrainer.
Even bigger differences arise if you do not set your trainer intensity to 100% (or even 200% (!!!) in some programs). Then, the program calculates with 15% (meaning you go very slowly), but the program sends only 7.5% (or less) to your TrueBike (so you go faster, causing more air resistance, but you will climb more slowly).
Finally, the values are not comparable at all if you are riding in ERG mode. The external app uses only your power to calculate your speed, while the TrueBike or TrueTrainer uses your cadence, gear ratio, and the circumference of your rear wheel to calculate speed.
The conclusion is that if you see differences between your external app and your values, there is no reason to worry. Your average power and calories will be the same.