I'm saying 19th is probably your shipping date, i.e. when the device is shipped (leaves the warehouse) to your home and the 25th is when it'll arrive. To fulfill that estimated delivery date, it can't ship closer to the 25th or else it'll push your estimated delivery date out further.
Unless I'm incorrect (and feel free to let me know), the phones were shipped from China and are already in the US (and likely in multiple warehouses throughout every state). Dallas has multiple distribution centers (there's an Amazon center maybe 15 miles from where I live, for example) and the likelihood that it'd take 5-7 days to ship from an Apple/T-Mobile warehouse seems slim. Not saying you're wrong in what you said. Just that it would be odd for them to have the phone ready for delivery and yet hold off for nearly a week even if they have it in their warehouse.
It could be part of a 2nd batch of orders that arrive after the first get shipped on the 19th. I guess I'll see as the 19th comes closer if my date either gets changed or I eventually get a tracking number with some notice like "Out for Delivery". Either way, it's about 10 days away