Gmail Folder Inbox Sync Settings

[hi-tech34]

I am trying to have all my inbox mail show on my phone and am having trouble getting the correct sync setting. If I set duration to sync 5 days and set the inbox to sync 5 days it shows all mail but if I switch the inbox folder to sync all it shows half of the mail?
When I switch back to 5 days it shows all mail agian? confused as to what Sync all means.