I like your explanation of one facet of a huge multifaceted reality, Bathsheba.

I believe it's because when you experience the energy of the one who's passed your own sensory system recognises the energy 'signature' and processes into a form that is meaningful for you, based on your memories and experiences. Because the totality of spirit is too huge for the physical brain to really process all at once. I believe in reincarnation. So if my grandmother passed over then appeared to me, I sensed her energy, then my own body recognised that part of her that was familiar to me in this lifetime (ie Nanna as I knew her) rather than all the other forms she might have taken in other lifetimes.