Why is the 2TB FD via PCI-Express and 1TB FD NVM-Express?

This can be a bit confusing!

First lets review what a Fusion Drive is: The word 'fuse' give us a clue here, we are linking (fusing) two discrete drives within the OS so they are seen as one.

In this case a SATA III (6.0 Gb/s) traditional spinning hard drive and either a PCIe 2.0 x 4 lane NVM-Express blade SSD (in the current model) or in the previous model a PCIe 2 lane blade SSD.

iMac 27" 3.3 GHz i5 (5K, Mid-2015)

iMac 27" 3.2 GHz i5 (5K, Late-2015)

For a comparison here is a benchmark between them:

• 2014/Mid 2015 iMac 5K: writes at ~716 MB/sec and reads at ~766 MB/sec.
• Late 2015 iMac 5K: writes at ~1520 MB/sec and reads at ~2080 MB/sec.

While the benchmark shows the faster 4 lane PCIe NVM interfaced SSD is faster than the older 2 lane PCIe. The width of the connection has a lot to do with it (2 vs 4 lanes).

To be clear here you can't use the newer SSD in the older system the system does not offer a 4 lane PCIe interface.