If you have the double tap to screen on function enabled, that's the most likely culprit. Once I disabled mine, battery was as good as advertised.
Thanks - I actually have that off already, learnt that mistake early
I should add - battery is as good as advertised, when things are working as intended - i.e. the sleep 'app' (and of course ensuring wifi doesn't remain on in sleep, another early learning...). Just some of the power management is a bit odd. I guess I'll just have to get used to using button on screen - but I think most tend to pause their music with the side button and just take headphones off. I'd fully expect in that scenario for Android's auto sleep to kick in at some point however.