I think his Last Name at Birth is just fine as Taipaleenmäki, which was the Finnish last name of his father. The father died, the mother married Johannes Persson Kitti, whose name does not have anything to do with Frans William.
The name Magnusson seems to be what Frank picked to use in Canada. It's not his own patronymic, he is the son of Abel, so should have been Abelsson in that case. I did check a bit to see if he was from an even earlier marriage with a Magnus, but found no indication that this was the case.
I don't think there is any way to know why he fancied Magnusson. Many times people just seem to have picked their name in the new country out of thin air. At other times one can discern a logic - an sometimes there is hardly any change at all. But this looks like a "thin air" case.
