I believe the Playsite Scrabble only covers the the international US/Candian Scrabble Dictionary (the TWL98 official words list). I'm not sure about the Yahoo version, but if memory serves me correctly I believe their version uses the same dictionary.
If, like my human, you are living in Australia, New Zealand or the UK, you may be more used to playing using the SOWPODS offical Scrabble words list. If you play using the SOWPODS Scrabble words list, you should download Networdz Scrabble which can be played through IRC. Networdz also supports the TWL98 dictionary. See http://www.scrabbleon.net
My human often plays in the #sowpods IRC channel under the nick of BlackDalek.