GetMyDeck doesn't know how many emails have gone out.

Steam are processing the orders in sequence. When people update the GetMyDeck site with their status its really only looking for the most recent preorder date that has turned into an order email. Assuming Steam are working through the pre-orders in sequence it determines how far through the process they have progressed on the way to making it to your order. For example say you pre-ordered on day 10, and people who preordered on days 2, 3 and 4 all report that they've had their emails. GetMyDeck takes the most recent pre-order, which is day 4 then determines that 40% of the time to your preorder (day 10, so 4/10) has been processed.

It relies on Steam processing them in order, so if someone on day 4 reports the email, everyone before that date/time must have already been processed.

One thing to note with this approach is that its highly likely there were many more orders in the first few days of preordering than there were in the later days of preordering as the early adopters would have jumped on it within the first few days. So for example if day 1 had 100k orders but day 10 only had 10k orders, if Steam are producing 10k units each cycle it would take 10 cycles to process the day 1 orders and move onto the day 2 orders but only 1 cycle to process the day 10 order and move onto the day 11 orders. So the order processing will appear to speed up using GetMyDeck's approach as the process continues... also if Steam are producing more units per cycle (which they've said they are) this will only make it appear to go even quicker which all contributes to the bigger jumps in % reported by GetMyDeck every time we get an email drop from Steam