If the main and PV sources are at the same end of the bus, the combination of all of the load breakers combined can pull a load greater than the carrying capacity of the bus.
There is, fortunately, no NEC limit on the sum of the branch and feeder breakers in the panel. That means that the bus protection has to on the supply side.
(Note that this does not apply when you have an MLO main panel with up to six service disconnect breakers.)
The real question I never see answered is why they chose 120%, since in theory you could have each end supplied up to the full bus capacity without ever having an overload at any single point on the bus.
I suppose that you would never be allowed to install a load breaker whose capacity is greater than that of the bus, but if you did, then the current limit for a single stab could become a problem.