Everything agglomerates one way or another when you are sweeping up a primordial disk of dust and gasses. Once 'separated out' into "bands" the agglomerates are still not well-behaved, they still keep on bashing into each other and slipping into each others backyard.
If near Oort objects fall sunward obliquely they are quite likely to end up orbiting in the Kuiper belt for a while before being hit on by something else in the crowd and so be shoved further inward. Not to mention that they are so slushy that they probably have been exchanging bodily fluids for a few hundred million years while there and are quite mixed up by the time they get picked up and get a going over by Rosetta and her future decendants.
Surely, isotope ratios must depend upon specific histories of specific objects, especially in a system that is driven by nuclear interactions and characterised by large but varying distances and over an immense period of time. Oort may not be representative of the original solar disk that condensed out the sun and all the other bits. It seems reasonable, given planetary composition variance with distnace from the sun, that it is a distillation of the lighter elements and compounds thereof. Kuiper less so, and so on back in toward us rocks sitting in the sun's highly irradiated atmosphere. Other objects even further out than us are not protected as well from alot of that irradiation. What do we know of how that effects all the assumptions the good doctor from Rosetta? Just because we have no data doesn't make a factor a negligible given. BTW: my STATS01 course suggested that a minimum decent sample size for saying anything about a population is at least 40. The sample of 12 referred to reminds me of a typical macro economic model built upon a set of 12 years of National Accounts (anything longer and the behavioiral game has changed for sure!) accepted used by gullible polticians to justify a major policy shift demanded by their commercial sponsors. :-)
Lets not allow cosmology and space science become another 'dismal science' for want of thinking a bit more deeply before adding to misguided premature debates about things being this way or that way when we are still in the data collection phase.... journalistic demands for 'statements' and desires for '15 minutes of fame' not withstanding.