Quite the opposite, your thinking of valid items that are so various per location is crucial and quite on point! I myself, can turn an Anthill into Mount Everest by thinking of more and more possibilities
, but in this case, you are not confusing the matter at all.
A series rated system should yield an amount determined at the load, through a device rated at that determined level at that point, but what happens if that device is exceeded! Now we're looking towards the next device.
Then there's the way the fault makes contact. Small - fragile conductors that have a relatively poor contacting surface produce low level faults. They usually trip the OCPD by means of a timed overload, rather than overcurrent.
Medium size faults can come from any number of situations and can be so unstable that they could keep from tripping an OCPD for more than 10 seconds! They could also become larger with locally generated contributions, or with better contacting areas. This is one of the problems with 4 wire Wye systems that are 1000 amps and larger, with higher than 150 VAC to ground [480Y/277 VAC 3 phase 4 wire Wye].
High level faults will almost always exceed capacity of frames sized 225 amps and lower, rated for 240 VAC [typical 10KAIC @ 240VAC frames]. If they don't blowup, they just have their contacts welded closed.
One bad thing [plus something to consider] is that most multi pole devices [2 and 3 pole breaker frames] have their capacity rated as L-L-L, making the per pole [single pole, or L-G] level extremely low. A good solid ground fault of medium size can exceed the capacity of one pole, causing failure to trip [or explosion - hopefully!!].
Another contributor to fault levels is, of course, Induction Motors - there's a local source of fault current that flows from the motor, to the transformer, then in most cases, back again through the system!
Nevertheless, motor contribution would be an almost impossible level to give an absolute sum to. The general way is to use the LRA, which comes close enough to what a typical medium to heavy loaded induction motor will develope when the system voltage drops to near zero and the current density becomes limited during a fault situation.
Completely different stuff happens to lightly loaded or no-loaded induction motors.
Not too sure what Commutator motors will do [DC motors], but they should contribute some level of DC back into the system.
I see that you stated in your message using a value of R from transformer to some point. That looks like the Ohmic method of fault calculations [as opposed to point-to-point].
Is that what is used in the mentioned calculator program, or is this your preferred method of fault calcs?
Keep me up-to-date!