Cells filled in without Auto-Fill, formulas, or AI--what did I misunderstand?

I might just be dumb but I can't figure out what happened today. I'm working on a quarterly report where I use a downloaded .cvs file as the bones. I clean the data, insert columns needed, and in some of those columns, I put enter formulas, do the whole autofill thing, normal stuff, right? I've done this report several times. My report has over 2000 lines of data that I have to check manually because the formulas aren't perfect. It doesn't take super long, and after a day and a half, I've gotten through 1400 rows. The weird thing happened when I got to the 1400s. I noticed a row I hadn't gotten to had data in a column with no formula, correctly formatted and accurate. When I kept scrolling, the entire rest of the column was filled in correctly without me editing it. There was no formula to auto-fill, and I don't use Copilot AI. I didn't copy and paste anything, none of that. I hadn't looked at that part of the sheet yet. It's probably silly, but it kind of freaked me out. This spreadsheet isn't shared with anyone, I'm the only one with access. The initial report is just data that I then break into chunks and send to someone else for the next stage.

Google only showed me Auto-Fill and Flash-Fill as explanations, and I didn't use either. I looked in the settings and couldn't find any permission that matched, and I don't know how to explain this besides ghosts, I guess. At the end of the day, it's less work for me, so I'll take it, but if anyone can kindly explain what I'm missing that makes this make sense, I would appreciate it!