Selection of Breaking Capacity in Circuit Breakers
The "6 KA" number (literally, 6000 amps) is based on being able to trip out at the maximum possible bolted fault (i.e. not having its contacts welded shut by the current).  There's really no point having a 25KA breaker when the delivery wires are incapable of delivering even 6KA of flow.  

The current capacity of the delivery wires is decided by their size and Ohm's Law.  Any particular size of wire has a known resistance per length, and this can be multiplied by length to give the resistance of the service drop from the transformer.  That resistance, and the service voltage, plugged into Ohm's Law gives the maximum possible current.  

For instance, some American power companies guarantee that their delivery drop wires will be sized so as to render current flow above 10KA impossible.  Thus, 10KA main breakers are legal in those services.  Since European services deliver twice the voltage (and presumably half the amperage), we can guess they use smaller service drop wires, and thus, have a lower maximum KA. 

This has nothing to do with the voltage on the transformer primary.  If 13KV gets into your house's electrical system, that's what your grounding and bonding system is for. The 13KV is unlikely to flow 6 KA because it is coming off rather lengthy feed wires that have come for miles.  The resistance of those feed wires will be much too great for 6 KA to even be possible. 

This isn't about lightning. Lightning is Thor's hammer, the Hulk can barely stop it, no molded case circuit breaker can.   A high voltage line like 13KV will have lightning arrestors, because otherwise, the lightning will find somewhere else to arc to ground, and when it strikes that arc, the 13KV will sustain the arc indefinitely.