Problem with calculating server usage for given tenant/project

Hi,
I'm trying to calculate servers usage for given project within a certain period of time. We have a special policy requiring calculating half of the resource usage between stop/start actions and none between shelve/unshelve so I have to calculate everything by hand and cannot fully rely on existing calculation methods.
I'm developed a simple script in python for this reason.
The thing/problem is:
1/ Firstly I'm trying to get a list of servers usage using nova.usage.get method for given period of time (let-s assume 2017-09-05 00:00:00 - 2017-09-09 23:59:59). The returned list isn't equivalent to nova.servers.list which seem to be ok at first glance. But it's not.
For given period of time from usage class I get a server with following info.
{ 'ended_at': '2017-09-05T13:08:59.000000',
    'flavor': '...',
    'hours': 1.3166666666666667,
    'instance_id': '...',
    'local_gb': 40,
    'memory_mb': ...,
    'name': 'SERVERNAME',
    'started_at': '2017-09-05T11:49:59.000000',
    'state': 'terminated',
    'tenant_id': '...',
    'uptime': 4740,
    'vcpus': 24}

It can be noticed this server ran for about 1 hour. For this reason I would like to check details and actions which are matching this server. So I ran nova.instance_action.list for this. The method is relevant to single command-liner (nova instance-action-list SERVERNAME) which returns:
ERROR (CommandError): No server with a name or ID of 'SERVERNAME' exists.

2/ Secondly, nova.servers.list returns me server which is not listed by nova.usage.get but it should have been. instance-action-list SERVERNAME returns the following actions on this one:
+----------+------------------------------------------+---------+----------------------------+
| Action | Request_ID | Message | Start_Time |
+----------+------------------------------------------+---------+----------------------------+
| create | ... | - | 2017-08-31T08:59:22.000000 |
| reboot | .... | - | 2017-08-31T09:20:13.000000 |
| shelve | ... | - | 2017-09-05T12:57:37.000000 |
| unshelve | ... | - | 2017-09-18T11:41:36.000000 |
| shelve | ... | - | 2017-09-18T12:31:30.000000 |
+----------+------------------------------------------+---------+----------------------------+

Why is this server not reported by usage.get method in provided period of time?It shoud return some info regarding this server but it's not. Isn't it a bug?
Alternatively - what am I doing wrong.....

Best,
Damian