it's not magically attacking at double speed. A round represents 6 seconds of time, during which you and your opponent are parrying, feinting and attacking. at the end of that 6 seconds, the d20 abstracts all of that action into one chance of doing damage or not.
if your enemy does something stupid, like turning around and twerking, you get a free chance to hit him with your weapon. One might call this an attack of opportunity or something like that.
This explanation would have made sense if:
- "Doing something stupid" that provokes an AoO included, for example, attacking another adjacent enemy - since "realistically" that also leave you at a diminished capacity to defend. TDE emulates that by the way, by only allowing one parry per round.
- A combatant engaged in melee with enemy A wouldn't get AoOs on enemy B - since "parrying, feinting and attacking" enemy A supposedly requires all their concentration;
Seriously, appealing to "realism" in a TB system with hit points is beyond stupid. If you want to argue for AoOs, argue for their mechanical value, don't pull this "realism" shit.