Warschau is the German spelling for Warsaw, Poland.
If he sailed from a German port (e.g., Hamburg, Bremen) then the German clerk likely used the German spelling of the place name.
The Resslince spelling is not a Lithuanian one for the place name.
Have you ever used the Slownik Gazetteer. See http://stephendanko.com/blog/1587
Slownik lists thousands of place names (using the Polish spelling) including many in Lithuania.
What do the U.S. Census entries for them list as their countries of birth?