definitely a design choice; the entire design of p2 seems to center around having minimal variation between different characters of the same level. Not just attack rolls, but also stuff like defenses, ac, skill rolls, there's very very few ways to modify them, and they're all tightly regulated. No more stuff like skill focus to get a +3 to one skill.
So the expected values for say, a lvl 10 rogue, will be very similar no matter how they're built.
It also makes me wonder though: where's the 11? with the default rules you can't get 11 in a stat. The numbers are chosen for purely legacy reasons; but odd numbers below 18 basically don't exist in the new stats system. Seems odd to use numbers like this but then ignore half of them. For us long-term users we know it exists for legacy reasons/compatibility; but for a new player, one might wonder why the intervening numbers can't exist.