The impression I get is that some items are flagged on their database as "we've got loads of those, and they are easy to find", so they say immediately [most recently over a hour before their 7am opening] "ready for pickup".
But when you get to the store they do the same as for a walk-in customer and go rummaging around backstage.
Fewer items are actually pre-picked, and left in a little bag on the "collections" shelf.
I've never yet had an item they had over-sold, leaving me empty-handed, but I seem to quite frequently buy the only one they [sometimes having to try three nearby stores] have in stock, so it's helpful if they reserve that for me until I can get here.
Having said that, the hard-to-pick item I mentioned above was an example of "last one in stock".