The assumption is that if you're actively in the channel, you saw the item already and don't need a duplication notification on your phone for the item you just read.
To expand on this, the desktop clients keep you active for much longer than say the web client. I was also having these issues so I asked for a clarification from their support.
I stopped using the desktop client for this reason and now I have a separate window open for the web client. Push works much better for me now.
The assumption is that if you're actively in the channel, you saw the item already and don't need a duplication notification on your phone for the item you just read.