The zl sub object finds only once the sequence 5 5 in this list: 1 2 3 4 5 5 5 6 and at position 6 (instead of 5). I'm not the best at counting, but the result for position is something disagreeable, from my perspective.
Matching appears to be done starting at the end, and every match is then removed from the list. Another possible outcome would be that zl sub would find two matches, at positions 5 and 6.
Example: according to zl sub the string www.zlsub.com does not start with the sequence ww.