Time difference in TICK script

InfluxDB 1 Kapacitor
1

I’m downsampling data to15m using TICK script, but script not querying at exact 15m slot

Raw data

> SELECT "SensorId","2002","2004" FROM Reading WHERE time > now() - 1m LIMIT 5;
name: Reading
time                 SensorId 2002                 2004
----                 -------- ----                 ----
2018-07-25T06:00:07Z 13867    0.15211888122558595  0.6903297305107117
2018-07-25T06:00:07Z 16474    0.009301428794860839 0.04855547845363617
2018-07-25T06:00:07Z 13852    222.72334375         373.1100769042969
2018-07-25T06:00:07Z 16432    18.61921501159668    34.59856033325195
2018-07-25T06:00:07Z 16419    16.664958984375      27.60993766784668

TICK Script

batch
    |query('SELECT mean(*) FROM "mydb"."autogen".Reading')
        .period(15m)
        .every(15m)
        .groupBy(time(15m), 'SensorId')
        .offset(1d)
    |influxDBOut()
        .database('mydb')
        .retentionPolicy('autogen')
        .measurement('Reading_Agg_15m')
        .precision('s')

This TICK script will run at every 15 min, query 15 mins data of yesterday, calculate mean and store it in Reading_Agg_15m measurement.

So for an instance at 2018-07-24 12:45:00 tick script will run and query data from 2018-07-23 12:45:00 to 2018-07-23 13:00:00, but when I checked in influxdb it’s showing sliglty different time range.

> show queries
SELECT mean(*) FROM mydb.autogen.Reading 
WHERE time >= '2018-07-23T12:45:06.169978273Z' AND time < '2018-07-23T13:00:06.169978273Z' 
GROUP BY time(15m, 0s), SensorId

Any explaination why time selection moved ~6 second?