That...is the dumbest way they could've set up those mechanics. Grar.
It actually makes a lot of sense, imo, since it allows a tank to Guard a target, shunting damage off of their ally without rendering their tank stats meaningless. The problem, as I see it, is actually in the programmatic complexity of doing such (my guess would be that it is accomplished by applying 50% outright DR to the Guarded target and Guard causing the Guarded player to clone all attacks and direct them at the Guarding tank since Guard damage is considered damage from a player). It's an excellent execution, imo, if a bit strange for most players to cogitate, but it makes the most sense from an applicable/balance standpoint.