There are many reasons the names could seem different:

1)Rebecca was the mother, but her husband/baby's father died/left her and she re-married to John either while pregnant or after the baby was born.
2)Someone wrote the surname sloppily and it was then mis-transcribed.
3)The family had a falling out and Abraham decided to use a different last name.
4)Abraham was adopted.

Sometimes a researcher will try to fix incongruencies by "making" it right the way they see fit rather than to leave facts/information as they stand. As far as I know "HILLMAN" is not an alternate to "ALLE/AN".