does anyone know why the primary shard gets most (95%) of the queries?

mongostat shows that the primary mongod of the primary shard gets 400 qps and the primary mongod of the other shard gets only around 20 qps

shard key is

and queries do not use the shard key, but data should be uniformly balanced on both shards
current chunk ratio between these 2 shards is 703 : 704

this is a test environment, so nothing else is running on these servers

if i "movePrimary" to another shard, then that's the one that will get most of the queries..