What must you reckon with here?

You answered correctly. Here, you must expect that the vehicle in front might change lanes and that traffic could become denser beyond the underpass. These are typical situations when approaching an underpass, as the view is limited, and more vehicles may merge or slow down after the tunnel. The right-hand lane is not ending, so that choice was not relevant. Always be alert for sudden changes in traffic flow and possible lane changes, especially near underpasses and in areas with restricted visibility.