Ranks in Nanoha are more utility and raw power based then a accurate representation of fighting ability.
Nanoha needs to move away from the ranking system. Power levels get annoying and it's better to just accept she's hax to the point her opponents aren't even sure if she's human.
Hayate comes top of the ranking based on her huge magical reserve and wide range of destructive and utility spells. Unfortunately she's terrible at duelling one on one (to the point she doubts with she could take on any of the StrikerS) and isn't too good at quickly processing and aiming her magic, a job she hands over to Small Boss. That said, she's a strong leader with experience with a dedicated team combined with the fact she's a artillery piece in of herself.
On the same note, Signum isn't ranked as high as Nanoha but can match her in a duel. Makes sense really, Signum specialises on duelling, she fights people one-on-one and has hundreds of years combat experience. Unlike Nanoha combat is pretty much her sum, Nanoha's higher ranking represents a wider range of skills combined with her gross raw magic strength.
It's not quite at the DBZ stage where they rely on a numeric value to say who is more likely to win, it's getting there but at least they've pointed out a number of times that people like Chrono who had experience and skill could defeat characters who are stronger based on stats alone. At least, he could before Fate and Nanoha got 10 years of combat experience in a time skip while he got promoted to a desk job.