Why do different aggregate functions return different timestamps?

#1

The first two queries return two records, yet the third and forth queries only returns one record. This happens since they return different timestamps. Why isn’t the timestamp the same for all results? When including functions min, max, sum, count, mean, and integral, all returned the same timestamp (the earliest date) except integral which returns epoch.

> SELECT integral(P56) AS integral_56, mean(P56) AS mean_56 FROM L5 WHERE time >= '2017-08-30T06:54:01+00:00' AND time <= '2017-09-22T10:27:21+00:00'
name: L5
time                 integral_56       mean_56
----                 -----------       -------
1970-01-01T00:00:00Z 5401831.388294995
2017-08-30T06:54:01Z                   7.35705237879954

> SELECT integral(P56) AS integral_56, mean(P56) AS mean_56 FROM L5 WHERE time >= '2017-08-30T06:54:01+00:00'
name: L5
time                 integral_56       mean_56
----                 -----------       -------
1970-01-01T00:00:00Z 5401831.388294995
2017-08-30T06:54:01Z                   7.35705237879954

> SELECT integral(P56) AS integral_56, mean(P56) AS mean_56 FROM L5 WHERE time <= '2017-09-22T10:27:21+00:00'
name: L5
time                 integral_56     mean_56
----                 -----------     -------
1970-01-01T00:00:00Z 21044076.564443 4.140744659132201

> SELECT integral(P56) AS integral_56, mean(P56) AS mean_56 FROM L5
name: L5
time                 integral_56     mean_56
----                 -----------     -------
1970-01-01T00:00:00Z 21044076.564443 4.140744659132201
>