【教えて下さい】「as was usually the case」の意味

Sadaijin (Minister of the Left) usually served as ichinokami, however, in the case where the position of Sadaijin was vacant or Sadaijin served concurrently as Sessho Kanpaku (Sessho Kanpaku was not qualified as ichinokami), Udaijin (minister of the right) was appointed as ichinokami.
すなわち、通常は左大臣が務めるが、もしも左大臣が欠員もしくは摂政関白を兼ねる（一上資格がない）場合には右大臣が任じられることになっていた。 - Wikipedia日英京都関連文書対訳コーパス

http://ejje.weblio.jp/sentence/content/as+was+usually+the+case