I guess it is Random luck...
My buddy spent 6 10draws(60seal scrolls total) and got Sage Naruto...
I spent the same ... 60seals and got Ao...
^
To me...it might be random luck with the rare you get BUT there seems to be a SET NUMBER at which point you can get A rare...

"Spend a certain amount of scrolls for a chance at a rare"
"The rare you get is entirely random"
^ Basically this lol

We could say that there's another factor...maybe it IS entirely random UNTIL you spend a certain amount in which case it becomes 100% guarantee for a random rare(such as Sage Naruto/Ao/PAINS)

With so many people saying a certain number of scrolls to get certain ninjas...and so many people agreeing after having tried it...my friend and I as well...
All we can say is that we don't feel it's entirely random lol :D Plain and Simple.