Getting skills past rank 5

[Captain Windy]

I am very confused currently. I have rank 5 full sail on the sailing skill and just got a chart that increases it by 3 ranks, making it rank 8. I assumed that this would mean a longer effect duration, but when i tested this, that was not true.

Rank 5 full sail, without the item seemed to last around 22 seconds even though the skill page says it only lasts 20 seconds.

Rank 8 full sail, with the item seemed to last around 22 seconds. Exactly the same as without the item.

Please could anyone tell me if skills are unchanged above rank 8 or if i'm misinterpreting or if i'm doing something wrong. Thank you.