Spells that don't interrupt

I know this isn't down to particular spells because sometimes the same spell will be interrupted, sometimes it won't.

For example, fighting Bassilus just now and was sure I generally interrupt his rigid thinking spell (his first cast), then use a potion of free action if I cannot interrupt his hold spells next. This time, hit him with an arrow during casts of both and he has pain animation but spell is still cast.

I've noticed this since vanilla but have never understood it... as I understand a spell should be interrupted 100% of the time if the enemy has the pain animation.