Non-expected results with boxplot's "factorSeparator" option

Hello,

I expect that the following code plots a line between the boxplots when the factor n°2 changes, but it does not (it works as expected if 'factorseparator' is set to 1). Do you know why please?

data=[1:20];
grp={[ones(1,10) 2*ones(1,10)],[ones(1,5) 2*ones(1,5) 3*ones(1,5) 4*ones(1,5)]};
boxplot(data,grp, 'FactorSeparator',2);

It gives that results:

Non-