Singular/plural string combinations were stored as individual strings in Drupal 7 and before, even though they form a unique key for translations in Gettext .po transport files. Also, when not importing .po files, Drupal itself stored the singular and plural forms separately without any relation to each other (the relation was only established, when a .po file was imported with the strings in question). Therefore we moved to a model where the singular/plural pairs are stored as a single value and translations are tied to that (and also stored 'serialized'). This does not affect the use of t(), format_plural() etc. from the Drupal codebase, it only affects the internal storage.
This change also made it possible to edit singular/plural translations altogether on the built-in locale module UI. Further improvements to that UI are still being worked on at the time of this writing.
For more information on the internal changes see http://drupal.org/node/532512#comment-5679184
