The real easy way to understand culverts and fords for OpenStreetMap is about 
the layers they are on and this dictates the nodes they use.

For a  ford the stream/river is at the same level as the road (effectively 
layer=0) and therefore they are able to share a node.

Because a culvert (layer=-1)  is not on the same level as the road but passes 
underneath so it cannot share a node with the road and therefore the culvert is 
attributed to the river/stream with a node either side of the road.

With a bridge the road (layer 1)  is not on the same level with the 
stream/river so again cannot share a node and therefore the bridge is 
attributed to the road with a node at each end of the bridge.

Hope this will be of help in understanding the problem.


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