There are differences, big ones. Mostly in the quality of the polling. Back in '48 the polls were heavily biased towards phone owners (read: the rich) where Truman had massive support amongst the poor and the polling was stopped well before the actual election because the result was seen as a foregone conclusion.
On top of that the person running against Truman ran an extremely low key and passive campaign with little to no policy purely with the aim of not alienating voters rather than actually gaining them.
The only real similarity I can see between the two elections is that in both of them one candidate is heavily behind in the polls, however there have been plenty of other elections with that situation and the majority of them don't end up with a comeback win.
