In my experience with using the Field Calculator, I have observed the following:
i. When calculating all records in a field, simply make sure no record is selected/highlighted.
ii. If the calculation is to be applied to only selected records, then, it becomes necessary to commit any unsaved edits before proceeding with the calculation. For some weird reason, if edits have not been saved, the Field Calculator does not handle edits pending in the cache properly. However, once Edits are saved, the calculation is automatically applied to only the Selected records.