As to why, I've never encountered a software implementation that precisely matched its design paper. Usually has a lot to do with publication deadlines. Papers don't change, software does.
No, the upper byte of key should have 256 possible values. It now has one (all zeroes, essentially). Keyspace is reduced to 1/256 of original, a reduction of 255/256.
I think his point is that 2 raised to 192 is 256 times larger than 2 raised to 184, meaning that you are getting 1/256 as many possible keys by reducing the key length by 8 bits.
As to why, I've never encountered a software implementation that precisely matched its design paper. Usually has a lot to do with publication deadlines. Papers don't change, software does.