The difference? According to Merriam-Webster’s online dictionary:
extemporaneous: “impromptu . . . carefully prepared but delivered without notes or text”
spontaneous: “arising from a momentary impulse . . . developing or occurring without apparent external influence, force, cause, or treatment”
Therefore, unless you went home and suddenly blurted what you wish you’d said at yesterday’s meeting, your Table Topics answer was extemporaneous.