There is an attack animation associated with attacking. (or "swing speed/time")
If an enemy moves away while your character is in this attack animation, your attack will miss. The same is also true for enemy melee.
Why...? I guess it adds to the gameplay that you can dodge certain attacks and your attacks will miss, so you'd have to think (a little) before you just random decide to swing. (besides, it lets you dodge enemy melee attacks, and kite those really fast mobs)
As for Frost Breath, I'd take a wild guess and say that it's a problem with the AOE.
Intended or not, well, I'm no Runic dev/spokesperson.