I think those guard breaks are more for aesthetics than to function as an actual guard break. You gotta admit some of those attacks would look funny without a guard break animation. I agree its weird for a guard break to be negative on block but ya.
Raansu understands it correctly.
Basically it's as he says. It's due to animation reasons. The animations (for various reasons) need to be that much recovery, but having them in a block stun for that long means the defender feels like his inputs aren't responding.
An easy example of this is Akira's
which is +0 on guard. However, the punch has a humongous recovery when free cancelled. So if you guard it and try to attack you'll see you're still guarding, it gives you the impression it's like +13 or something but he's only +0 and it's waiting for his recovery to finish before letting you begin to attack.