Shard duration 24h and only 2 shards were created after a week

My retention policies:
name duration shardGroupDuration replicaN default

autogen 120h0m0s 24h0m0s 1 true

And i have strange shards groups. I see only two of them.
38 - seems right shard with 1 day duration, but “28” have duration of 7 days. And its seems nor right

id database retention_policy start_time end_time expiry_time

28 tickdata autogen 2017-12-11T00:00:00Z 2017-12-18T00:00:00Z 2017-12-23T00:00:00Z
38 tickdata autogen 2017-12-18T00:00:00Z 2017-12-19T00:00:00Z 2017-12-24T00:00:00Z