I think it has to do with its initial calculation of your BMI. If your pre pregnancy BMI is considered overweight, it adjusts how much weight gain is "normal" but doesn't adjust the weekly development.
BMI isn't a reliable or accurate measure of healthy weight though, so as the poster above said, if you feel good and your doc says its all good, no worries.
Yes, it always said above average for mine too, but my weight gain was usually spot on in the middle of the average range. As long as you're feeling good and the doctor isn't concerned, don't worry about what the app says!
That's the same reason why I stopped entering my weight, but according to the article, it's what I should've gained.
It is always saying I'm above average so I quit putting mine in because it made me feel fat.