My guess is that it is due to a bottle neck in the T2 chip. The two iMac Pro modules striped should theoretically give you theoretical read performance as high as 7000MB/s and write performance as high as 4200MB/s. Now there is a bottleneck of the PCIeX4 interface of 4000MB/s. There is also going to be bottleneck in the T2 chip that receives reads/writes from the PCIeX4 bus and stripes them out to each drive, while additionally encrypting and decryption the data with its hardware AES encryption engine. My guess there is a read performance bottleneck in the T2, that is worse than the write performance bottleneck.