>>davidb+(OP)
The "Texture Healing" feature is a really smart use of OpenType features to make problematic monospace combinations look much better without breaking the grid at all.
One naive way to do this would be to create ligature pairs for difficult pairs (mi, lm, etc). But instead, they seem to be selecting character alternates that fill the fixed width differently based on their surroundings.
>>albert+XF
The issue with that is that it would reduce the space between pairs that need it (such as mb). Taken to an extreme, mm would run together into one letter.