Those are honestly insane. I just posted a thread similar time as you did, my 6S 16GB just scored about 60MB/s and 900 MB/s in that test.
For comparison, my 256GB Samsung 850 Pro SSD gets around 400MB/s write and 500MB/s read. For a mobile device that is insane (each sector is smaller on the iPhone which probably plays a role... but still). Will probably return it for a Samsung 950 Pro M.2 drive when it releases (1.5GB/s write and 2.5GB/s read )
My 6s plus 64gb gives different results for every storage test. Generally speaking write is around 350 Mbps, read is btw 500-1200 Mbps. I guess it depends on status of system, processes running background etc. apart from that the app may have a bug.