Inserting a new value in a sequential key is always going to go into the 900 node. When that fills, you can add a new node and just balance the parent branches (assuming postgres doesn't try to keep a proper balance across the nodes too?).
However in the random tree a new value could be inserted into _any_ of the leaf nodes, which means you either have to create more leaf nodes than you require (in order to leave plenty of blank space, and even if you do that you will probably end up with uneven fill, which means some searches will be slower than others) or you end up having to refactor all of your nodes every time you insert a value.