Since Office 365 was created Microsoft basically updates your Excel regularly. So the Excel 2016 that you started using a year ago might NOT be the same as the one on your desktop today. Assuming you connect to the internet and allow 365 updates, you get new options all the time, what can be confusing sometimes - so, no you're not dreaming Excel is changing :-) .
Let's look at some of the latest changes now, so you'll understand what kind of updates are happening.
A few months ago a bunch of new functions suddenly appeared out of nowhere (MAXIF, MINIF, ...) and during the 2017 summer the Add-in button (Microsoft Store) suddenly moved from the Insert to the end of the Home Tab, while the previously activated Add-ins got a separate button on the Insert Ribbon Tab.
Excel 2016 Before
Excel 2016 NOW
(Previously : Store)
(Previously :My Add-ins)
However, if an Add-in can only be loaded once in a file (like the Stock Connector), it will appear in a separate Add-in Tab.
Another recent visible change is the Autosave setting appearing in the Quick Access Toolbar:
AutoSave is enabled if you work on a file that is saved to OneDrive, OneDrive for Business, or SharePoint Online. If the file is saved to another location ( like on a SharePoint On Premises site, a file server, or saved to a local path like C:\), then AutoSave is disabled.
The new update philosophy is great, however when you are the one that has to make manuals, screen captures, it's not cool at all. It 'll give you a lot of extra work (and a lot of stress too).