Try restarting your browser. If the tags have previously been removed, they might still be cached on the website view.

As for where they came from, you should be able to tell by selecting them. They might be automatic tags, which come from keywords on various sites, LoC subject headings, etc. You can disable those in the General pane of the Zotero preferences.

I checked and have not turned off the display of automatic tags. In the Standalone I see the tags as I have selected/organized them, whereas on the website, things like 1980s, 4th-ammendment, 6th-ammendment, acetaminophen appear (this is a natural resource database!). There were a couple times where I tried to import a reference and something went wrong, and a ton of non-relevant references were imported. I deleted them, but its possible they came in that way.

However, it still doesn't seem right that if you've deleted them, have managed your tags and you are displaying automatic tags in the Standalone and don't see them there, they shouldn't appear on the website, right?

Turning off importing automatic tags for the future isn't a good solution. Occasionally a reference will come in with none or with super generic tags, and in that case I edit them. However, most of the time a reference comes in with the author-designated key words, which we want. It would be super time consuming to have to enter those every time, so turning it off all the time won't work either.

At any rate, the issue is not whether to import them or not, its why the tags edited in the Group Library in the Standalone are not the same in the online version.

You're misreading what Dan says. It's possible to turn off the _display_ (not import) of automatic tags in Standalone. That could explain the discrepancy.
Also, if you have items in the Zotero trash, their tags would likely still appear online.