this is what i understand to be true but maybe someone else can verify this.....
short answer, theres no difference, neither k/o has likely been certified in equipment to carry currents.
250.92 says to bond around both concentric and eccentric knockouts with bonding jumpers at "services" regardless of voltage, then 250.97 says for over 250 volts to ground, metal raceways and cables with metal sheaths that contain any conductor "other than service" conductors have to be bonded same as in 250.92, except for option(1) because that would be bonding equipment to the g.c. on the services load side downstream.
i've used a listed 4 square box with only one eccentric k/o that doesn't need to be bonded around because the locknuts can make good enough contact outside the eccentric ring, but have to be sure its listed for that.
my understanding is that like reducing washers, the k/o's in gutters and panels are never acceptable to carry any currents so they should always be bonded around with bushings and jumpers, except for the type i mentioned above.
