why does it say "available to order", and not simply "available".
This looks like a pre-order announcement.
a. Their announcement right here
is a pre-order announcement. You'll be able to order in the future. The sky is blue too.
b. If the orders on day one strip supply it won't be available. They may not know how high demand is going to be but indications are that demand outstrips supply.
... It could also just be an allusion to most of the orders being BTO, especially if they are configured and assembled in the US.
Not just BTO. There are several factors in play.
1. EU markets have been without a "buy new" Mac Pro from almost 11 months. There is a high probability there is a sizable demand bubble there.
[ Sure some folks have bolted to other solutions by now but not everyone. ]
2. Apple invoked the Osborne effect almost 18 months ago. Again another demand bubble generator. For example, a quite sizable number of users on older Mac Pros put themselves into a holding pattern even though they had resource availability to upgrade. They won't because has essentially told them something newer was coming so they stop. That is classic Osborne effect.
[ Sure some fraction of folks have bolted to either 2012 models or other options that are either in the church of box-with-slots form factor or have need for dual CPU packages. The two latter aren't 'new' movements. ]
3. Apple stopped selling the 2012 model well over a month ago. Worldwide in the same boat the EU markets have been in for 11 months. At very least replacing a healthy fraction of those who were already in the "waiting pattern" but bolted in the mean time.
All of those can contribute to perhaps a run on the standard configs also. There is probably some "speculative" filling of inventory by resellers too. Pull in a pool, so that if a drought they can move them at full prices. [ exactly why many of them have been running around getting pre-orders lined up. Get a count and pad it with a 'safe' number. ]
Behind all this is probably a factory that has a top end production rate of 6-8K/units per month. Largely because past the initial "Sooner land grab" and 49er gold rush frenzy they probably don't need a factory with a higher annual run rate.
Once all the deferred purchases are gone, for the overwhelming vast majority of the year simply need a factory that can match the demand of the new arrivals that want one.