why is $RANDOM so non-random?

The following short script prints a random ten-digit binary number:

#!/usr/bin/zsh
point=''
for i in `seq 1 10`
do
    echo $RANDOM >/dev/null
    point=$point`if [ $RANDOM -gt 16383 ]; then echo 0; else echo 1; fi`
done
echo $point

However, if I remove the apparently useless echo $RANDOM >/dev/null line, the script always prints either 1111111111 or 0000000000.

Why?