Using older versions of Outlook for Mac for personal email and calendar
I don't think this is quite true...
It seems that the contents of the inbox are lazy-loaded and it's only showing the number of emails that have currently been loaded - not the total number in the inbox
For example:
I have an inbox "On My Computer" where the tooltip says "0 items"
I click on the inbox, and some emails appear
The tooltip now says "25 items"
I scroll down the list of emails, forcing a load, and the number reported in the tooltip increases as more items are loaded