SO has some lines with ship set and also scheduled

We are on 12.1.3

We have a SO created from the OM interface (source is another application ).All the lines are in the
entered status and 2 out of all the lines on the SO have ship sets 1 and 2 respectively assigned to them.Other ines
have no ship sets .All lines were created together.the 2 lines with ship sets are also scheduled(we allow scheduling
in entered status).

The questions we hhave are:

1.Why would only 2 lines have ship sets and not others and how these came to have ship sets(which setups would determine
that only some lines have ship sets and not others on the same order ).All lines have teh same line type