As I posted a little earlier (
link) I have similar results with my MSI order, "Date ordered: 17/09/2020 15:16:50", order number 58738xx, which the calculator puts me at 2651 (which based on the totals estimates on the spreadsheet I think means my order number was generated when the email was sent, not even when my payment was synced with the online store sometime between when I last looked Saturday and first looked Sunday morning).
Hopefully OC are using the Date ordered data they also clearly have (since it is also in the email), and if so (
@Gibbo can you let us know what data you are usinbg for determining who is "First come, first served"?) the calculator should either be shifted to use that, or just have a warning that in such cases it is very inaccurate.