For lanes defined from frame elements, the moving load is applied directly to the frame elements that were used to define the lane. This true for zero-width lanes as well as nonzero-width lanes.
To obtain more realistic load distribution to all girders, you may need to define the lane from layout line (this requires the bridge modeler license), specify lane width to match the width of the structure, and make sure that the group loaded by lane (on the lane definition form) contains all frame element representing the girders.
Lane