I think you’re misunderstanding how active and replica data is managed.
Your bucket is split into 1024 shards or vBuckets, and of you have 1 replica you’ll also have 1024 replica vBuckets.
All active vBuckets will be equally distributed across the available nodes (so 512 each with 2 nodes). Same for the replica vBuckets.
When it comes to keeping recent data in RAM, priority is given to active vBuckets, if there isn’t sufficient RAM to keep all data resident. This is why you see 100% of active data resident in RAM, but only 40% of replica data.