I believe your correct.
nsMemhit is the amount of data read back from NVram and/or DIMMs. This would be data that was written in the last few seconds and then requested for read.
nsSSDHit is the amount of data reads from the SSD pools. So the total read cache hit ratio is nsMEmhit + nsSSDHit.
From the stats above, i would guess that the data is large sequential blocks. Large sequential blocks are not moved into SSD cache because the Nimble arrays can read large sequential blocks just as fast from SSD or the SATA pools.