After creating a geometric network there are additional vertices at line intersection of existing data, which did not exist before.
In ArcGIS 10 the behavior of creation of a geometric network with Snapping has been changed. At intersecting lines a vertex is created also without connection in the geometric network.
When creating the geometric network, snapping must be disabled. In case of migrations (e.g. using FME) of network data to UT for ArcGIS therefore should be paid attention to the required position accuracy of lines and net points. Thus later the networks can be built with deactivated snapping, resulting in no additional vertices.
Esri's BUG entry for "NIM076675 - Vertices are added at intersection of features after building geometric networks" is unfortunately no longer publicly viewable.