ServiceNow tables commonly use table extension, where a child table inherits fields from a parent table.
When a field is defined on a parent table, that field is automatically available on all extended child tables.
Because of this inheritance model, deleting a field that originates from the parent table effectively removes it from every table that inherits it, which can appear as "the field was removed from multiple tables." That behavior is characteristic of an inherited field: it is not independently defined on each child table, so it does not exist as separate copies to delete individually. A reference field is simply a field type that points to another table and would not inherently remove itself from multiple tables when deleted. A custom field is typically created on a specific table; deleting it would usually impact only that table (unless that table is a parent and the field is inherited, which is precisely the inherited-field scenario). "Base system field" is not the right explanation for multi-table removal; the key cause is inheritance through table extension.