I think I understand now why we can't guarantee that the lowest post number in the original graph is a sink (it may have outgoing edges to vertices that have already been visited before it).

But why is the case different for reversing the graph and looking at the highest post number? Why is this a foolproof way to find a vertex in the sink SCC of the original graph?

I think we reverse the graph for the following reason: this is a stack operation.when we reverse the graph,we get the sink SCC for the reversed graph firstly and we push it to the top of the stack...we repeat the operation and finally the graph is null.then we pull the point set from the bottom of the stack,which is the sink SCC for the original graph.