It updates very frequently so I'd imagine its fairly accurate. Bare in mind it's based on working days (not including weekends and bank holidays) which there have been lots of with Xmas and new year.
My guess is that this relates to if all items are in stock and not marked as pre-order when you are ordering as there are some people waiting on orders over 28 days old where the graphics cards are concerned.
Usually they are very fast if everything is in stock.
It's based on actual numbers so it's as accurate as the input data. I have no idea whether they're including builds waiting for stock deliveries in those mumbers though.
When you are selecting it through the configurator, it should either have "(pre-order)" written next to it, or be slightly greyed out - so you can still select it but it just means a possible delay.
Then when you click Proceed, it does a quick check and you may get a warning then too.
The only definite way to find out is to ask them directly as mods on here are just volunteers so have no access to stock lines / quantities etc or orders.