Getting incorrect response from vCenter Server while fetching HostSystem performance data using queryPerf in VMWare infrastructure API

While querying the performance data for HostSystem using queryPerf method of Performance Manager of vCenter server, I noticed the following behavior, which is not expected.

Configurations:

Library used: vim25(API)

vCenter server version: 6, 6.7

Here are the code snippets and description for the same:

where host_mor stands for the ManagedObjectReference for the HostSystem entity whose performance data is being retrieved. Variable metrics consists counterId of the metric to be retrieved.

Issue ONE:

start_time="2019-06-12T12:56:19.000Z"
end_time="2019-06-12T12:59:19.000Z"
queryspec = Connection.vim25client.new('PerfQuerySpec', entity=host_mor, metricId=metrics, format=format_type, intervalId=20, startTime=start_time, endTime=end_time)
perfdata = Connection.perfManager.queryPerf(queryspec)

Expected behavior:

The vCenter should return the perfdata which will have performance data for the following time range:

First sample: 2019-06-12T12:56:20.000Z

Last sample: 2019-06-12T12:59:00.000Z

While the actual output contains the performance data for the following time range:

First Sample: 2019-06-12T12:56:40Z

Last Sample: 2019-06-12T12:59:20Z

Here, the sample for time 2019-06-12T12:56:20.000Z is missing and sample for time 2019-06-12T12:59:20Z is returned, though it is greater than the end_time.

Issue TWO:

start_time="2019-06-12T10:28:23.862Z"
end_time="2019-06-12T10:31:23.862Z"
queryspec = Connection.vim25client.new('PerfQuerySpec', entity=host_mor, metricId=metrics, format=format_type, intervalId=20, startTime=start_time, endTime=end_time)
perfdata = Connection.perfManager.queryPerf(queryspec)

Expected Behavior:

The vCenter should return the perfdata which will have performance data for the following time range:

First sample: 2019-06-12T10:28:40Z

Last sample: 2019-06-12T10:31:20Z

While the actual output contains the performance data for the following time range:

First sample: 2019-06-12T10:28:20Z

Last Sample: 2019-06-12T10:31:00Z

Here, the sample for time 2019-06-12T10:28:20Z is collected even it is before the specified start_time and the sample for time 2019-06-12T10:31:20Z is missing.

Can someone please look into this and suggest if I am missing anything or it is known behavior.