Because it sorts by string comparison. If you have a string $a, and another string which is $b = $a . $any_other_string, than the shorter string $a will always sort lexicographic before the longer string.

If you look into a phone book, you expect Strauss to come before Strausse, so that's what you usually want.

The only case where this doesn't do what you mean is to when you compare numbers of different length. That's what Perl offers the <=> operator for.