Thanks for the insight. It seemed like some had much less grain than others, which didn't make sense but I suppose some will have received more noise reduction than others. Also the softness I mentioned on some of the other photos didn't seem like missed focus; they had a specific quality - watercolour effect, like you say, which now you've mentioned it I do recognise from when I've gone overboard on noise reduction when post processing digital images.
It's odd as well because one of the grainiest images was the cat, which I really liked the effect of! Surely you choose a film knowing and presumable accepting (or wanting) the level of graininess that comes with it...