A helmet is there to slow down and dissipate the energy of an impact to your head.
It should do this in as predictable/uniform manner or , how a manufacturer decides what is best is up to them (design deformation materials etc) but if its certified it has to go through the relevant tests (SNELL, ANSI etc).
Obviously the bigger the impact the greater the damage - which may or may not be visible
I had a spill last year when i had a meeting with a car and my old Giro looked fine, yet a lows peed fall offroad in May left a huge crack in the front of it's replacement.
Both did their job IMHO.
There are to many variables involved in predicting should or should it no have split, Speed of impact, rider weight, angle of impact etc. possibly met design them too fail in a specific way like crumple zones on a car that fail in a predetermined way for a given force.