Unable to test what you’re asking about since I’m 100% Xert 
but if you have a FIT file with calories from another source you could post it to Strava, note the number there, then import the activity into Xert and see if same figure or recalculated.
Here are related posts on how Xert calculates their number –
Calorie calculation, data bars, MPA calculation - Support - Xert Community Forum (xertonline.com)
Xert’s calorie calculation does equal KJ? - Support - Xert Community Forum (xertonline.com)