What’s the deal with ‘to order’ Christmas food?

All of my shopping is Tesco home delivery, so I booked my Christmas slot the day they became available. I’ve gone back in today to add my Christmas to order food and half the stuff is showing as ‘unavailable’ - it’s not because of my delivery date, so does this mean they are out of stock already? I thought the whole point of ‘to order’ is so that this doesn’t happen?

Not sure about everyone else, but I can’t have my full Christmas menu locked down by the middle of November.

Can anyone explain this to me please?