There are several possibilities.
1. When people come through the U.S. immigration center, they are asked their names. Many years ago, the immigration official wrote down the name as he/she heard it. This generated many new spellings of old names.
2. Some immigrants simplified the spelling of their name on purpose, to make it sound more American to to make the name easier for Americans to pronounce. Obviously, some Polish letters don't even exist in the English alphabet, and so can't be spelled in English! I wonder, for instance, if in Polish the second letter of Dabrowski, that "a", had a "tail" under it. This makes the "a" into a different letter, that has a "swallowed m" sound.
3. Sometimes there were variants in the country of origin. My grandfather Waclaw Frontczak came from Poland in about 1908. When I have gone back to Poland to visit relatives, I have found that some relatives spell the name "Frontczak" and some spell it "Fronczak", without the "t". Even they can't tell me when this change occurred!
Susan Marie