Considering their 20th anniversary Digivice pre-order page only lasted a day, it seems they're allocated a set number of stock to begin with rather than waiting until a certain date and then ordering stock. Could be wrong though.
Speaking of Megatron, I was super lucky to get him. Went in-store to pre-order, staff wasn't able to find him on their system so they rang head office and was told I had 15 minutes to order from the website before the page was going to be taken down.