Henry, I'm a little surprised your Zone 1 density is the same for N-1, N, N+1 rating the film at EI 250 in all cases. TMY is a straight line film with a short toe so Zone 1 tends to move as development times are altered. Just a thought.
For what it's worth----with my tests for TMY and TMX Zone I densities do not change appreciably until the +2 and -2 dev time----in both cases about a 1/3 stop off the measured speed, I'm not surprised at those results. With T-grain film, I consider 1/3 of a stop to be an appreciable difference.
"The difference between a very good print and a fine print is quite subtle and difficult , if not impossible, to describe in words."
---AA (The Print)