To understand why there is a difference between the implications 705.12(A) and 705.12(B)(2)(3)(a), consider the following.
Panelboards for loads in general, are routinely populated with more breakers than the total busbar rating, and more breakers than the main breaker rating. For instance, you could total the loads on a 200A panelboard, and see that they add up to 400A. You could even total the operating amps of all the connected loads, and find that it also exceeds 200A. If someone wanted to go out of their way and turn on every single load that they possibly could, they would overload the panelboard, and trip the main breaker. The underlying assumption is that this is an improbable event, and that not all of the loads draw their full rating simultaneously. There is diversity to the timing of when each branch circuit draws its full load, and there is a load calculation procedure in the NEC based on statistical models of user behavior to design your feeder and service sizes such that they are realistic for a likely total current draw, and this tripping the main is indeed an improbable event.
It goes without saying, that improbable does not mean impossible. Ordinarily, the fail-safe from exceeding the busbar rating of the panelboard, is that the main breaker trips before the service conductors and busbar get overloaded. Connect a PV source to that panelboard on the load side of the main, and it will add up with your main breaker, so that there is a chance that you do exceed your busbar rating with no fail-safes to protect against it. Connect on the same side of the bus as the main supply, and the current on the busbar will exceed the busbar rating when this happens. Connect on the opposite end, and we get to take a little bit of credit for the fact that Kirchhoff's current law works in our favor, as current is subtractive in opposite directions, instead of additive. Not full credit for Kirchhoff's current law, as you might optimistically think, because there are other factors that come in to play, which is why it is an industry compromise that it is a 120% rule and not a 200% rule.
When the interconnection is supply-side of any given panelboard's main breaker, the main breaker doesn't care that the power comes from multiple sources. The breaker protecting the panelboard will stop the total current from exceeding the busbar rating, regardless of what mix of sources supply the panelboard. This is why with supply-side interconnections per 705.12(A), you don't even need to think about protecting the busbar; the main breaker of the panelboard already does that for you.
While not explicitly stated, I see an issue with 705.12(A), on whether the limit should be the ampacity of the SERVICE CONDUCTORS is your limit in addition to the ampacity of the SERVICE. These are not necessarily interchangeable. Your service conductors might take credit for the 83% rule on a residential service, or the 240.4(B) next-size-up rule on a commercial service. Given 4/0AL service conductors on a 200A service, I would recommend a conservative approach, and a limit to 125% of your total system current to 180A, rather than 200A. Likewise, given 2 sets of 500 kcmil CU service conductors on an 800A commercial service, the conservative approach is to limit yourself such that 125% of your total system current to 760A, rather than 800A.
