As Mutsu is telling Mikogami about this concept, he classifies both Minato and Mikogami as being "special" in that regard, referring to Minato's ability to wing Homura, a previously thought to be un-wingable and biologically unstable Sekirei. He then draws a comparison to his own Ashikabi, using the fact that Mikogami managed to "reset" and wing another previously thought to be un-wingable Sekirei in Akitsu.
The explanation is likely not a simple matter of mathematics or science (genetics) however as it's not mentioned whether Mutsu sees this sort of occurrence in Natsuo, the Third Generation Disciplinary Squad's Ashikabi and co-leader or in Higa, The Ashikabi of the East. No mention is given to Seo Kaoru, Sanada Nishi, or Yukari Sahashi either, despite their status as Ashikabi with above-average or distinguishable prowess. This leads to the potentiality that this is a very rare phenomenon, even amongst Ashikabi, an uncommon breed amongst humans.
To support and explain this concept, it is noted and shown in many instances that Minato makes time for each of his Sekirei and cares deeply for each, treating them as people, not mere weapons or servants, despite his tentativeness towards embracing the romantic feelings that his five adult Sekirei have for him. It's also noted and shown that Mikogami treats his Sekirei more like rare collectables. However different the approach, both would have one thing in common with the other, some manner of deep emotional connection to or investment in their Sekirei.
In comparison, Higa and Natsuo begin with detached, strictly platonic and professional relationships with their Sekirei which only begin to grow sometime after Higa's loss of a good deal of his Sekirei and Natsuo's reflection on his relationships with his three Sekirei, predominantly his with Karasuba. These two don't however notice the positive emotional change until during the last battles on Kamikura Island, although Natsuo's views towards his female Sekirei will likely never manifest in exactly the same way as other Ashikabi due to his sexual orientation.
Additionally, while Seo, Sanada, and Yukari do appreciate and love their Sekirei, their Sekirei lack the extraordinary or unique characteristics that Homura and Akitsu once possessed, thereby making them less likely to qualify as having yet developed a Core, if they were to at all.
-Minato Sahashi (exemplified in his relationship with Homura)
-Hayato Mikogami (exemplified in his relationship with Akitsu)
- Sekirei Manga chapter 169