Lets imagine that key.interestOps() in thread 1 is called so captures the value 0, then thread 2 is scheduled, calls key.interestOps(), captures value 0 too, then thread 1 is scheduled again and calls interestOps, now interestOps has a value of 4 (write). Thread2 is scheduled and calls interestOps, now interestOps has a value of 1 (read).
So at the end we should have 5 we have 1, interest in write has been lost.

This scenario is possible, it has been observed reading traces.