I believe this is a combination of things mentioned above

United is not just catering your flight from insert hub to insert destination. The are ordering for dozens of flights and this is hundreds of meals (this is really rough numbers throughout, I'm not going to count). The pre-order window is up until 24 hours. They aren't asking Sysco or whoever to send XX pounds of beef to the ORD caterer 24 hours out. All of that if much further our in terms of raw materials getting ordered and delivered to the kitchen.

So what they are probably doing, using really general numbers is: we are estimating 15 beef meals on each Intl Polaris flight ORD-EUROPE. So let's say 30 flights x 15 beef = 450 beef meals.

However, we will open up pre-order for 450 beef meals. Let's say everyone on the ORD and CDG flights order beef. UA is still prepping 450, but now there are none left for the other flights on that day. I think there are overall estimations and targets, but they aren't limiting per flight, but flights with the same menu.

I'm sure the math and projections are way more complicated than this, but I'm sure United is looking at this holistically, not just what they are doing on UA 929 today. e.g., My sister used to do projections for options/accessories on General Motors vehicles. She wasn't look at one specific dealership when she ran those projections.