The table user_lastaccess has all accesses saved, not just the last access. In File blocks/online_users/block_online_users.php just the last 20 accesses are selected, so if one user has 20 accesses, just this user is shown. To correct, change line 76 from:
if ($pusers = get_records_sql($SQL, 0, 20 )) {
to
if ($pusers = get_records_sql($SQL, 0)) {