Obviously as nihonto is an art and not a science there will be some degree of subjective opinion and fluidity... I am still learning the Juyo 'game', but I agree with Darcy in that certain schools and smiths are more predominantly featured at Juyo level simply due to the fact that if a mumei blade of a certain style reaches a certain quality it will be attributed to the 'top' smith/school of that style - the Norishige/Tametsugu-Sanekage/Uda/Shimada argument for example makes sense.That is why you see Juyo attributed to the first 3 smiths in that list, but very rarely the last 2.
So therefore unless you have a signed example of a 'lower' tier school it will be less likely to pass Juyo, as any high quality mumei blades by this school would already have been attributed to a higher tier school of that style. I guess you could call it a sort of selection bias at the H/TH level, but there is a certain logic.
My 2c