I notice in reading the introduction to Attic provided in Pharr's text (I'm looking specifically at §465 on page 201 [244 of the PDF] now) that there are some morphological phenomena that were pulled from Homeric into the Koine, while others came instead from Attic. Specifically, I notice that ε + υ in contraction in the Koine follows Attic and becomes ου, while in Homeric it becomes ευ (§461).
However, like Homeric, the Koine has σσ where Attic has ττ (as in, θάλασσα v. θάλαττα). Also like Homeric, the Koine has ρσ where Attic has ρρ (for example, in the word ἄρσεν).
How did the Koine receive these forms? Why didn't it follow Attic?