HP correlates more with the song length, followed by note density and where the map is denser.
HP 5-6 can be a nightmare for a marathon, while it's a free pass for a short map (0:30-1:00). And we're talking about Oni/Inner Oni difficulty level. What would one expect for low difficulties for short songs? Yes, a free pass too. Low difficulties are there to introduce the game and teach the basics, not to give them free passes.
Another example with HP 5-6. Let's say we have two maps. Same number of notes, same HP, same length. But one is only denser/harder than what you can play in the beginning, while the other is only denser/harder in the end. What would be easier to pass? Yes, the former would be, why? The latter will drain your HP harshly in the end, risking your chances to pass the map, while the former would let you fill up your HP bar steadily after passing the heavy HP drain section, securing a pass.
These are some factors you have to consider when selecting a suitable HP.
And yes, this is a guideline as one can't define a number, not every Kantan/Futsuu needs a high HP, long songs/dense maps would need it lower. Same for Oni and above, short songs/low density maps would require a higher HP than usual.
HP 5-6 can be a nightmare for a marathon, while it's a free pass for a short map (0:30-1:00). And we're talking about Oni/Inner Oni difficulty level. What would one expect for low difficulties for short songs? Yes, a free pass too. Low difficulties are there to introduce the game and teach the basics, not to give them free passes.
Another example with HP 5-6. Let's say we have two maps. Same number of notes, same HP, same length. But one is only denser/harder than what you can play in the beginning, while the other is only denser/harder in the end. What would be easier to pass? Yes, the former would be, why? The latter will drain your HP harshly in the end, risking your chances to pass the map, while the former would let you fill up your HP bar steadily after passing the heavy HP drain section, securing a pass.
These are some factors you have to consider when selecting a suitable HP.
And yes, this is a guideline as one can't define a number, not every Kantan/Futsuu needs a high HP, long songs/dense maps would need it lower. Same for Oni and above, short songs/low density maps would require a higher HP than usual.
So why ozzy did this 
