Queries on Mongo Secondary are not equally distributed

I have a MongoDB cluster setup with one primary and 2 Secondaries in a single shard setup. Every night we have a high load but one day and most of the read APIs have a SECONDARY_PREFERRED read preference.

But the load not balanced as shown below: enter image description here

X-Axis -> time Y-Axis -> number of queries

Green -> Secondary one Yellow -> Secondary two

And the CPU of the corresponding machines enter image description here

X-Axis -> time Y-Axis -> % of CPU IDLE

Green -> Secondary one Yellow -> Secondary two

We are using Mongo Java Driver Why would the query be distributed unevenly?