When Tom addresses the audience, his speech is much more erudite and lofty than when he simply talks to Amanda, Laura, and Jim in the play. His age difference and his experience gleaned throughout the years easily account for this discrepancy. Although the exact time lapse is unknown, Tom is definitely a more mature man than he is in his role as a breadwinner living with his mother and sister against his will.
Form is also determined by function. Whereas the speech throughout the play is natural, spontaneous dialogue, Tom's language to the audience is more like 'a speech' or public address. The form Tom chooses is a deliberate one; he takes a certain poetic license when narrating the story. (The true "Shakespeare" in Tom finally comes out!) In the opening lines, he explains that unlike a magician, he is representing fact as a kind of fantasy, modulated and toned even as memory is never really an exact "replica" of the true event it represents.