if u set the true landing alt at the selector. the airplane will do the math and find out the differencial pressure between FLT ALT and LANDING ALT(the one on the selector) and compare with the pressure here:
below FL280 7.35
280-360 7.8
above 360 8.35
and if the if the actual diff is lower than the one according to the cruzing level.the airplane will set the cabin alt abit lower than the figure on LANDING ALT SELECTOR so she only withstand a lower diff pressure .
the airplane is pretty smart
but higher cabin alt means less oxygen to us.so we played the trick and fooled the airplane by set a number lower than the real one .
this can explain why 6000 not the real landing alitude .