Good points Ariel. 
 
It's fascinating that the sword/saber/knife/dagger/etc continuum of weapons seems so prone to this kind of linguistic weirdness.  It shows up in other weapons (notably halberds), but outside of weapon terms, the best example I know of is the shrub vs. tree pair.   
 
I suspect that swords, knives, and daggers get into this because there are issues of violence, power, and utility all coming into play.  A knife is a fundamental tool, but a military knife is also a symbol of power and maybe that's why they get different names?  Just a guess. 
 
Best, 
 
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