Default Insert Behavior on Sharded Cluster

Hi - I’m just looking for possible confirmation of an observation made recently. When browsing through the monitoring console for our sharded clusters I noticed a pattern where traffic for inserts was consistently higher on shard-0 ( which is our default shard) but relatively even for other operation types - query, update, delete… etc. I am assuming this is because at insert time in a sharded cluster the shard key is new and therefore not assignable to a particular shard based on configDB shard-key ranges. Am i correct in this? …or is this a symptom of some configuration shortcoming? … thanks for any insight you can lend.