Why $random is not controlled by seed?

There is a segment of code below:

for(int i=0;i<10;i++)begin

$display("current random value: %0d", $random);

end

I use Cadence irun to compile and simulate this code. Although I add option " -seed random" to irun, but each time the result is same. It seems that $random is not controlled by -seed.

If I change $random to $urandom, it works! Each time I run, the result is not different.

So, my question is that, why $random is not controlled by -seed ???

Thanks.