Confused by FICO

Quick and dirty: Citi shows my FICO score using Equifax data. My score fluctuates 15 points every month (up and down) despite no late payments and no new accounts/credit limits, etc. Recently, my score dropped 38 points. The only change was that 6 months ago, I requested all 3 credit reports and noticed a "negative account" which is a store card. That card was supposed to be closed in 2020 but the store never closed it. The balance was 0 for 5+ years. I called the store to close the account.

I spoke with Equifax and they informed me that closing an inactive account with 0 balance should not affect my FICO score. No other activity was found in my credit report. I spoke with Citibank and they had no idea on why the fluctuations occur and why the recent 38 point drop. They told me to call MyFICO. I called MyFICO and they have no information for me, except to tell me to review my credit report (which I already requested with Equifax).

So, my question is: why does a 0 balance account (5+ years) show up as a "negative account", and why does closing this account seemingly caused a near 40 point drop in my FICO score?