The Personality number sums the consonants of the full birth name, and the instruction to use the full birth name is doing more work than it appears to. It assumes there is one, that it is recoverable, and that it is the name that matters.
For a large number of people at least one of those fails. Adoption may replace the recorded name entirely, sometimes without the later name-holder having access to the earlier one. Registration practice varies: in some jurisdictions a middle name is recorded and in others it is not, so two people given identical names by their parents can have different documents. Names are misspelled by registrars and the misspelling becomes the record. Refugee and displacement documentation frequently reconstructs a name from memory.
Order matters too. The system was built for a naming convention in which a personal name precedes a family name, and it treats the first name as first in several positions. In the many cultures where the family name comes first, or where a patronymic sits between, applying the rule requires a decision that the rule does not describe.
Practitioners resolve all of this case by case, usually by asking the client, which puts the client in the position of choosing an input before the calculation runs. That is worth knowing when the resulting figure is presented as something given rather than chosen. Within the tradition the Personality is the least interior of the name positions, which makes the dependence on a document less awkward here than it is for the Soul Urge.