Question re Cast List - DVD credits v IMDb

I am mystified more by the credits than I was by the movie.

Comparing the cast list at the end of the DVD that I have just rented (in Australia, if that makes a difference) with the Cast List provided on IMDb, there are huge discrepancies. Actors and/or characters credited on the DVD do not appear on the IMDb cast list and vice versa. (And IMDb don't list them, as they sometimes do, as extra cast, presumably provided independently by the usually uncredited performer. And, even weirder, some are listed but with completely different character names/descriptions.

The most obvious example is Todd Ryan Jones. On IMDb he is the sixth actor credited - and he is listed as The Demon. On the DVD he is much further down the pecking order, being the 25th actor credited - and described merely as Tim's friend. (No one is listed as The Demon on the DVD - I have the credits frozen and have quadruple-checked!)

Mark Radfield is described a Middle-Age Man on the DVD and Ronnie's friend on IMDb (but IMDb don't credit a character called Ronnie, so why does he merit a friend??)

Camilla Zaidee Bennett is credited on the DVD as the little girl - an important if minor character - but not listed on IMDb. Katie Foster and Lauren Lakis are given character names by IMDb but listed as Female Friend 1 and Female Friend 2 on the DVD. Does IMDb know something that the production company don't?

And Erik Young is credited by IMDb as the Civil War Possessed Soldier. Maybe I am watching a different movie - I don't recall any possessed soldier, civil war or otherwise.

There are many other examples - this is but a selection.

I know that sometimes IMDb only lists partial credits - I'm not sure why. Presumably the production company could quite easily provide the full list from the closing credits. Is that what has happended here? Or have there been substantial changes in the version released in Australia? (I doubt that they have been that major, judging from all the other threads, reviews and parental advisories here).

What is going on? And was there a civil war soldier??

Thanks.