Magnetism as in the ability to attract and charm people, where as charisma is the ability to attract and charm people and somehow inspire devotion in them.
Sounds like you conflate charm with charisma there; the two are of the same root, while the latter has been tarred with a populistic connotation as of late.
(That said, quite a few of my favourite people would likely pass as 'profoundly un-charismatic' in a popularity conquest. Charm lays in the eyes of the beholder, y'see...)
Muhibija Buljubasic, 56, stands as his wife Senija puts cutlery on his body in Srebrenik, Feb 23, 2014. Buljubasic discovered five years ago that he had the unusual ability to attach items to his body by radiating a special energy. Without making any special preparation, he was able to hold on to spoons, forks, knives and other kitchen appliances, as well as non-metal objects like remote controls and cell phones. 
"What is it that makes some people more captivating than others, even if they say/do the same things in virtually the same way?"
Energy and enthusiasm. There seems to be a shortage of it among people so naturally to go towards it and when they interact with that individual they to get a bit of that energy. Even if that means just being spontaneously nice to said energetic person