qgis - Why does QgsSpatialIndex.nearestNeighbor(point, 1) sometimes return 2 lines?

I'm trying to get the closest line to a point using PyQGIS in a plugin. The active code file is this: http://pastebin.com/TDzmsKVM

When I request the closest line (only 1)

nearestIds = self.spIndex.nearestNeighbor(self.feat2.geometry().centroid().asPoint(),1)

I sometimes get a list with two lines and the first one is not the closest but as far as I can see the second one is. Why is this?

Does QgsSpatialIndex use only boundingboxes for lines? How do I get the real closest line always, not only most of the time?