Could be something to do with the only have Hyper deflect (or whatever the viral one is) and one other Hyper ability being activated at any one tme, but I'm not sure.
You can have two at a time, Viral Defense and one other, you can't have ballistic Melee and thrown on at the same time. You have to have a weakness, so that means if there is a gankfest going on you can only defend against one thing at a time. If your fighting one on one then you can choose which defense you need.
No buffs that buff the same stat ever stack (with the exception of one of the Aikido buffs I think). So Hyper-Dodge and Block don't stack with Enhanced Dodge and Hyper-Deflect doesn't stack with Network Firewall or Viral Shielding.
Now sometimes things act strangly: like a Hyper-Dodge/Block will block Enhanced Dodge. On the other hand Viral Shielding will overwrite Hyper-Deflect, but Network Firewall won't. Still, considering how UM and Imp. Hacking interact I don't think a higher Hyper-Deflect will ever be safe from bad Viral Shieldings .
Both Enhanced Dodge and Viral Shielding grant 48 base Defense(s), the same as level 50 base Hypers, but why is it different for one then the other? No clue on that one...
I noticed that enhanced dodge even deletes teh hyper-sense abiulity which doesn't even have thrown defense. Now I can understand the purpose of enhanced dodge is to save your teammates a few extra memory slots, but Hyper-Sense ? either thrown defense needs to be added to Enhanced Dodge or it needs to stop interferring with the hyper-sense ability.