I was looking at a chart and it seems something is not quite right, though I could also be wrong.
A screenshot is attached. (tried to attach the chart itself, but the forum doesn't like it)
Lagna at 17d Taurus, in Krittika. Mercury at 1:20 Gemini in Rohini, Mars at 1:44 Gemini in Krittika.
So it seems like either Mercury should be in Krittika, or Mars should be in Rohini. My swisseph calcuation program says Mercury should be at 13.2 degrees of Krittika.
I should have looked at my implementation of Dhruva GC more closely, because actually it does show what Kala shows, Mercury being behind in Gemini, but ahead in Krittika, going into Rohini first, even though it is still behind Mars...weird
I know swisseph has an ayanamsa labelled Dhruva GC (Wilhelm), but when I use that, Mars is ahead in the nakshatra as one would expect. When I use my own implementation of Dhruva GC, it shows Mercury ahead in the nakshatra and behind in the sign...so it must have to do with the coordinate transformation, perhaps with the ecliptic obliquity.