Why isn't damage done to Giblartar's personal shield counted on score screen?

@whutwatDamage doesn’t register correctly for badges/challenges, it seems if I do lets say 800 damage the challenge would say 700, it would always be 100 less for the challenges and idk why... it happened numerous times it would be my first match of the day so It started from 0, i would do 953 and the challenge would say 853