I will have to find my Kill-A-Watt and do some better testing... but neither my LM465 nor my PLM21 will dim to "zero", I can say that for sure. They hit a minimum threshold, and further dim commands are ignored.
By eye, both the LM465 and PLM21 appear to have 15 levels (counting the two ends, full bright and full dim). This is taking the double-step into account - there are really only 7 levels available to me.
Curiously though, I did some more testing with SDM3 and the 2414U, and found that if I hit the buttons on the HR12A *REALLY* fast, I could get SDM3 to report either THREE or sometimes just TWO repeats of the dim/brighten command! I could not reliably repeat this though, so I wonder if it is a timing/sync artifact, or as Dave mentions, a difference or irregularity in how the commands are interpreted by the Smarthome box (the 2414U). My other thought was that it could even be a case of poor switch debouncing in the HR12A itself, and not necessarily (or entirely) a firmware issue?
Dave or Brian, do either of you happen to have an older RT504 ("Sixteen Plus") remote, or the equivalent Leviton 6313 ("Hand-Held Wireless Controller")?
It is times like these that I wish I owned an oscilloscope.