I don't think it's randomly assigned; the time required to rest increases at levels 7, 13, and 19. It does this because of, I imagine, the 1hr + 1turn/level "zoo" spells being intended not to last longer than the time between rest periods without extend spell. If you think about it, the "zoo" spells last the full time between rests at level 6, 12, and 18.
I wish rest timers were reduced by 1 hour across the board, so that these spells could last one entire rest cycle... as ultimately, people just frustratedly sit around trying to actually do something typing
//1 hr
//1 hr
etc.