On a second look, seems like a combination of Alice not being good at encrypting (in a sense that it had to encrypt in a way that makes decrpytion possible), and Bob needed to get up to speed on decrypting (even with the key).
As soon as that relationship clicks, Bob get better very quickly, while Eve improves at a slower rate for a while, before Alice adjusts (causing bob to get worse for a bit, until it catches up)
Would be interesting to see a set up where Alice / Bob get trained first, then Eve is activated and see how they adjust.
This is probably because Alice wasn't good at encrypting yet.