The origin (0,0) is located at the top left in NetSim whereas in SUMO it is in the bottom left. Due to this, the SUMO network will be shown inverted in NetSim GUI upon importing.
The networks are using Cartesian, metric coordinates where the leftmost node is at x=0 and the node being most at the bottom is at y=0. This means that when being imported, NETCONVERT and NETGENERATE (SUMO utilities) are projecting the network, first, if the original network was not using Cartesian and/or metric coordinates. Then, they move the road network to the origin at (0,0). This is the reason why SUMO networks imported in NetSim are displayed closer to the origin (0,0).
Refer http://sumo.dlr.de/wiki/Networks/SUMO_Road_Networks for further details.