Here is a bit more information on this topic...it might help you to understand why the decision was made
The difference actually relates to the origin of these words. The words muster and mustang have no relationship at all to the original shortform of must. The word "must" actually has 3 meanings: be obliged, or as a noun it can mean new wine or mould (hence musty and mustiness are on the shortform list). The word mustard, in fact comes via the "new wine" meaning in that mustard was originally made using must or new wine when grounding the seeds of the plant.
- This reply was modified 7 years, 9 months ago by claurent.