Understanding the OSPD

[tahnan]

My brother has introduced me to the Internet Scrabble Club, where I've played a few games with moderate success. (A few rewarding bingoes, like opening with POINTED, or hooking the D in GROINED onto LAZE. Or the game against my brother where I managed RESTORE, with the first two letters played under PA and FAT, and then later, given an M on a triple word score from a previous turn, played MISFILED onto the adjoining triple word.)

But I'm hoping someone on my friends list is a serious enough Scrabble player to explain something to me about the Official Scrabble Players Dictionary.

Why the heck does it contain RETIGHTEN but not *RELOOSEN, the latter of which I had challenged off the board?