There is a lack of clarity regarding the difference between a channel message and a post message. At first I thought perhaps channel messages was a combined count of post and reply, but that does not add up when looking at the totals shown in my environment. (If it IS a combined count and there is just an issue with my reporting, I think that should be clarified since so many other metrics are stand alone and since there would then obvious be an opportunity for the numbers not to add up, leading to confusion.)
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@KazuyDarklight Thank you for submitting feedback and contributing to the docs. We are currently investigating this.
Is there already an explanation about this question?
we have the same one.
Hi @KazuyDarklight, thank you for your feedback.
I experienced the same behaviour:

@LanaChin Could you kindly help us to find an explanation for this? Thanks!
@LanaChin Hi Lana could you please help on this issue?
Looking into this
@LanaChin Wondering if there is an update here? Thanks!
Btw: What exactly means if the Display Name shows as - -?
Btw: What exactly means if the Display Name shows as - -?
It's an user that's removed from ur tenant
Thanks boyday94. But (at least from the view I have on different tenants) there are too many such entries for small time periods (e.g. 30 days), that I believe, that this is the only reason. How do you think about?
For example, I have here my Dev-Tenant with Contoso installed. Never changed User Accounts but still see, the most active user is the "--". For this case, it could be, that the activities were triggered via Microsoft Graph API.

I've pinged the PM for guidance on channel/reply/post messages.
@LanaChin @KazuyDarklight Channel messages does not filter to just include only post or only reply message types. In the definition of Post messages it qualifies as "unique post messages", whereas, the Channel Message definition says only "unique messages" and this _implies it includes all of the messages_, both the posts and also the replies. Similarly, for Reply Messages, it is qualified as "unique reply messages".
As per definition
• Channel messages is the number of
unique messages
that the user posted in a team chat during the specified time period.
• Reply messages is the number of
unique reply messages
that the user posted in a team channel during the specified time period.
• Post messages is the number of
unique post messages
that the user posted in a team channel during the specified time period.
It's helpful to include the "Reply Message" definition above. Now, from these definitions we can derive that Channel Messages will include the unique count of all the messages, including the Post and Reply Messages.
If you are not seeing that and have a specific example, please share back here in this thread. In meantime, we appreciate the question and will consider this question as we work to enhance our docs.
Thank you @quentinhurdazure! Closing this issue.
Most helpful comment
@LanaChin @KazuyDarklight Channel messages does not filter to just include only post or only reply message types. In the definition of Post messages it qualifies as "unique post messages", whereas, the Channel Message definition says only "unique messages" and this _implies it includes all of the messages_, both the posts and also the replies. Similarly, for Reply Messages, it is qualified as "unique reply messages".
As per definition
• Channel messages is the number of
that the user posted in a team chat during the specified time period.
• Reply messages is the number of
that the user posted in a team channel during the specified time period.
• Post messages is the number of
that the user posted in a team channel during the specified time period.
It's helpful to include the "Reply Message" definition above. Now, from these definitions we can derive that Channel Messages will include the unique count of all the messages, including the Post and Reply Messages.
If you are not seeing that and have a specific example, please share back here in this thread. In meantime, we appreciate the question and will consider this question as we work to enhance our docs.