When the application updates the validator set it calls updateValidators() with an 'updates' parameter, a list of changes that is then parsed in order, one entry at a time, making updates for the corresponding validator. i.e. the list is treated as a transaction list.
For example [add v1, rem v2, add v3] is different than [rem v2, add v1, add v3] and the call with one will produce different results than with the other.
This is an issue for gaia that requires these updates to be instead interpreted as a set.