The reason Team RWBY got an Affinity that was so overpowered was because of certain factors they all shared and their unique Mating.
In-Universe, several things about Blake claiming them when she did and certain similarities between them acted to increase the chance of them getting such an overpowered Affinity:
- First, all four are Huntresses-In-Training that are at peak or near physical condition, know how to use their Aura for their Semblances and are quick studies when Blake teaches them how to upgrade their Marks to the second level, so their Auras are similar enough that this helped increase their synchronization. Ruby IS slightly younger then the others, but not to a large degree and even if three are humans it's still close enough in biological terms.
- Secondly, Blake gave them all Stage One Marks very close together chronologically speaking, literally moments apart according to Ruby's recollection of receiving her own Mark. It's likely that in a traditional polyamorous Mating time is taken between Marking at least two persons, a shorter timespan increases Affinity chances when going to Stage Two since the shared Aura is "fresh" so to speak.
- Third, the fact that Blake was in Mating mode during the Marking. Since modern Faunus usually control their monthly cycles with medication it's likely they're making Marks while not in their cycles or under medication. As mortified as Blake was about the whole ordeal, her Mating with the other girls was probably enhanced because her own body was more receptive to giving Aura than she would had she been in a normal state of mind, i.e. dealing with the understandable doubts and fears she would face later.
- Fourth, the Semblances all four possess are active ones instead of passive and have to be used with clear intent. When using the others' Semblances it's shown their Marks light up with the corresponding symbol and color of the Mate in question; if one of them had a passive Semblance they had no control over, their Affinity may have allowed them to share it but not turn it on or off; having four active Semblances means their Affinity dictated they could access the others at will (but not without training.)
Taken all together, Team RWBY once again beat the odds when it came to getting an awesome Affinity because of these factors coming together.
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