I think it's random. But why no VATS melee? VATS increases melee' potential exponentially by like 10x. Without VATS it is pretty bad and well behind ANY gun's DPS due to the poor real-time melee combat implementation. After a few hours I couldn't take any more of it: Slow swing speeds, no crits, animation reset if (even jabbed by a no damage level 1 raider punch), 100% blocked attacks (even by a level 1 raider fists), and absurdly heavy staggers. Some high level enemies (who can 1 shot you even in PA) even if you block their attacks and make them stagger instantly are blocking again and can NOT be hit outside of VATS.
I guess you can make it work with Pain Train to prevent stagger/block locks, but still I wish they designed real-time better. I can't wait for mods.
It's just a manner of speech and usually in jest, https://www.youtube.com/watch?v=qYpx2KkDzSg