I think it has something to do with the actual reference price being a calculation, not simply the last trade, something about the last 30 seconds and VWAP: https://www.cmegroup.com/trading/equity-index/faq-sp-500-price-limits.html
It's really significantly different, not just 2 or 3 ticks. I don't know the reason but it's probably this:
Wow, following the links, ending up here: https://www.cmegroup.com/trading/price-limits.html#equityIndex this is what I come up with: Who knew there were so many dense tables of normally irrelevant information?
Did ES and YM go limit down again? It looked like there were plenty of orders coming in. -821 is only a little under 4%.
I detected this Sunday night and brought it up last night. I don't use graphs but somebody gave me the closing price at 3 pm central on the emini from last night and the 5% down from there didn't match up with 5% limit down. As long as the come gives a reference price you can follow in these crazy times. Its very important as if you trade after hrs you can get stuck. Based on were the spy is right now at 6;40 eastern it looks like the mini will open at 2350-2360 as the spy is right near its 52 low . All this can change by opening.
Limit down...if you want to see the actual drop watch the ETFs. Only way to see where the futures are as they are once again limit down and will be halted at the open