I think what happens is the grabaseat page is loaded in sections, and for the few minutes that it takes to update the fares it displays the sold out flag. It's a very strange way to update a web page, but maybe there is some link to the booking engine that requires this.

The problem is that early in the day you can't know if the route is genuinely sold out, or if they're just updating the page. So right now, an hour after your post, Auckland Whakatane shows 31 / 27 seats available - even though it was shown as sold out when you looked. It having availability makes sense too, because how popular could that route be? But CHC PMR shows sold out - which also makes sense given that both are university towns, and one of the key grabaseat demographic targets is surely students.

Basically if you fancy a route that is showing as sold out early on, check the page again five minutes later.