The list construct seems to look for a sequential list of indexes rather taking elements in sequence. What that obscure statement means is that if you unset an element, list will not simply jump to the next element and assign that to the variable but will treat the missing element as a null or empty variable:
$test = array("a","b","c","d");
unset($test[1]);
list($a,$b,$c)=$test;
print "\$a='$a' \$b='$b' \$c='$c'<BR>";
results in:
$a='a' $b='' $c='c'
not:
$a='a' $b='c' $c='d'