Short swords do 40% more damage than daggers on average (1d6 vs 1d4) and this matters less over time as strength is maxed and magic bonuses are applied. If this Roman was at high level, the two words were effectively interchangeable for him.

It is a "pugio", a sort of small dagger sidearm for Roman soldiers. I can't think of any reason to refer to it as a short sword in English as that conjures up the image of a much larger weapon. These are essentially the same as the daggers used to kill Julius Caesar, for example.

It's possible the author thought using "dagger" twice in two sentences sounded clunky.

Most of the complex classifications for swords is a modern development, that's often fairly arbitrary. Maybe short sword isn't the best term, but it is a short-sword

Agreed it looks way more "chopppy" than "stabby" to me.