A lot comes down to technique and how hard you press the sandpaper. A light rub with P400 gives a very different result to using it strongly pushed into the wood. Too much pressure with fine sandpaper and you can squash-up the ends of the wood fibres, making it harder for stain to penetrate and giving a patchy appearance. It can also vary by the type of body wood.

It is far more important for the surface to be level than for the surface to be ultra-smooth at this stage.