Some of them have a slot that does 1 point to SPA 0 (Health) that enables twin casting for those classes. The important difference between say cleric and druid is that the cleric spell has a 1 tick duration, making its SPA 0 slot a HoT and so preventing it from twin heal. It's a bit convoluted. The obvious reason to add SPA 0 slots is to enable twin casting in the first place. For clerics to have it (normalizing?) but then also put in the duration values to disable it is weird. Josh's bug report might get that looked at.