I certainly wouldn't rate the slate hone as a finer grit than the arkansas. It isn't about how smooth the stone feels, but how it affects the edge.
The slate certainly feels glassy to the touch, but it produces quite a dull edge. I built up a little slurry and honed for maybe 20 passes and then TPT. The razor was not sharp enough to shave. I few passes on the arkansas and the edge improved incredibly. I wouldn't use the HHT to compare these hones. For me it has absolutely no bearing on the sharpness compared to the difference in the feel of the edge on TPT or shaving.
Nick