$urandom not so random when working with "force" in Verilog

hello experts,

suppose $urandom always giving us different random values every time called in verilog but I noticed sometime it's not so random, i.e., "force" can almost freeze its random capability. e.g., see my displayed results after example code:

        $display("$urandom=%h",$urandom);
        $display("$urandom=%h",$urandom);

    aaa=$urandom;         $display("aaa=$urandom=%h",aaa);
    aaa=$urandom;         $display("aaa=$urandom=%h",aaa);

    force aaa=$urandom;         $display("force aaa=$urandom=%h",aaa);
    force aaa=$urandom;         $display("force aaa=$urandom=%h",aaa);

=================

$urandom=119e3f23
$urandom=617da7c2

aaa=$urandom=1782d52f
aaa=$urandom=4ac07d95

force aaa=$urandom=80010e00
force aaa=$urandom=80010e00

and if I vary different seed, the "force"-ed urandom doesn't vary too many bits, majority of its bits stay 0s.

----seed 2---

force aaa=$urandom=80021c00
force aaa=$urandom=80021c00

-seed 3----

force aaa=$urandom=80032a00
force aaa=$urandom=80032a00

not big deal, but very curious why "force" having anything to do with random? P.S., I'm using irun or xrun for pure (system)verilog, or even in VerilogAMS.

thanks,

David