When many fingers are resting on the glass it is very easy to influence which letter it goes to. Try it without all the supernatural hooha and the letters - a clean glass on a kitchen table with fingers lightly touching it skitters about like a nervous kitten.
You don't even need to do it deliberately - if it is wandering about and veers a bit towards, say, T, then some people think that is where it is heading so their eyes and minds go ahead of it, and their fingers follow. With one letter "picked out", some minds (it only needs one) think of the next (a vowel? an O?) and off it goes obedient to their thoughts and unconscious pressure. By then everyone who knew a Tom is thinking "It must be Tom!" and lo and behold - it goes to M.
When the next question is asked, there is an expectation that it works, and that "Tom" is ready to answer, so anyone with a Tom in their lives is primed to recognise whatever answer makes sense to them.
If one of the company thinks themselves psychic, or is feeling mischievous, there is even more chance of getting "messages".