NE doesn't have a lower chamber, just a Senate. In most legislative documents NE is left grey for the lower chamber. Also all Senators in NE run without party affiliation in a top-two primary. I assume that you are imputing the party of NE Senators from other sources.
Nebraska has a unicameral legislature. By tradition the members are referred to as "senators". It is the largest chamber for purposes of determining who may vote for Congress (and because of the non-partisan primary actually has an effect).
My point is that to legislative organizations like the
NCSL the unicameral legislature is an upper chamber. In national meetings the NE legislators are afforded the title and same courtesy as the senators of other states, so it seems like more than just tradition.