UAs pre-order system is not a true pre-order. UA has a set number of each dish they will load on a flight. What you're really doing is pre-reserving one of the allotted dishes. Once a certain percentage of the dish are reserved, it is no longer available for "pre-order". For those who did/not could not pre-order, the FAs will take orders based on priority. Therefore, if you don't pre-order and are low in priority, you're basically going to get what's left.

I'm still in awe how UA is doing this. When I fly DL, I have never seen a dish not available for pre-order.