What is the rule for StreamIndex of Random Number Streams in worker by default

I want to get reproducible random number sequence in parfor-loops and follow the example in Repeat Random Numbers in parfor-Loops. By the test

parfor i = 1:4
  str{i} = RandStream.getGlobalStream;
end

I find that when the number of parfor-loops is larger than the number of workers, the StreamIndex of the random number stream used in parfor-loops is not definite and often switch from several choices.

So what is the rule for StreamIndex on worker by default and how to control it?