True enough, Essex is the man whom Roxie boasts is the father of "Tom."  But
there's a notebook entry (sorry I don't have the source handy) showing that
in an earlier conception of the story Twain planned to have PERcy Driscoll
as the father of both his legitimate son and Chambers, the son of Roxy.
This would make the indistinguishability between the two infants more
realistically plausible.

Why he veered away from this to have Essex as the father of Chambers is
either beyond my recall or not known.