It may be preference, but I do think it's skill set, too. I have barely washed any dishes in the 25+ years of our relationship (only when he is out of town.) I don't do it well, I don't like it, etc. My spouse has both the disposition and the skill set for it--loading the dishwasher puts into practice all those Tetris skills--and he likes the linear nature of it--dirty, clean, and a clear-cut place to put stuff away.
Sorting through the mail? Kinda stymies him. It's the opposite of the dishes...it needs to be read, processed, thought about, decision made about being actionable (or not,) no neat place to put stuff away. I do the mail processing, because I have both the skill set and the disposition to do so quickly and efficiently--the opposite of me doing the dishing.
While we certainly can do either chore, there is both skill set and disposition involved in deciding who does what most efficiently, effectively, and enjoyably.
So, yes, sometimes, "I don't know how" is a cop out. And sometimes, some of us are truly better at some types of pattern recognition and problem-solving than others.