Obscure? Well yes. They do not appear in the nikayas - the most complete source of the buddhas teachings. You have provided chinese references, which is not my forte. You refer to them as sutras, again this will immediately set my spider senses tingling. As for "ten or more" this is an off the cuff remark ( I could have said 20 or 30 ) my point was the Buddhas teachings are generally reiterated again & again & again & again...............................
Moreover, to only accept those doctrinal elements which "appear again & again & again", rather than find those portions which are essentially the teachings of the "real, historical Buddha", it may be quite the opposite:
In the process of transforming those teachings into a canon to be recited, the appearance of stereotyped pericopes (standard, repeated passages) became more and more important. Moreover, it is just these very pericopes which are the most inconsistent across textual traditions. This is because, due to their stereotypical and oft-repeated nature, they could easily pop in here and there, without anybody noticing their otherwise incorrect protrusion. The texts are well known for the fact that as their compilation progressed, the addition of stylized elements, which appear "again and again" became more prevalent.
This is just one reason why texts and passages are not "counted", but are "weighed". It is not always such a sound criteria.
One criteria that actual experts in textual analysis do use, however, is "lectio difficile preferenda", "the difficult reading is to be preferred". This is because whereas during the process of oral and written transmission, it is very easy for passages to gradually tend towards the most common, the fact that an otherwise uncommon or unusual passage is retained is usually considered greater testament to its authenticity.