The app tells you the range for about the 50th centile, but every baby’s different:)
My 9.5mo baby is 7.2kg, she’s also very light. What’s important is that she’s been growing, and she’s been maintaining the centile range over the past few months. She’s just naturally petite, like my husband and elder daughter.
What you can do is plot your baby’s weight on the growth chart and see if baby’s growing and staying in the same range or dropping. The other thing is to ensure that baby’s drinking enough. Baby needs 150-200ml/kg/day. Latching for 5 minutes on one side probably yields