Because Apple specificity states "2nd generation" on the iPhone 6s/6s+ pages, and on the iPad pages and iPhone SE pages it just states "Touch ID Sensor" with no designation of being 2nd generation.Wait...how do we know this? I would assume it has the newest touch sensor.
It sucks having the older sensor in the iPad Pro but your muscle memory gets used to holding your finger there longer because of the size difference between devices.