Strange Step climb behaviour

Im not really sure if this is a problem or not but it seems a bit strange to me.When i today was flying from ESSA-LGKO with a ZFW at about 61 tons.My max altitude was about FL360, so i entered my cruise alt at FL360 , and entered step to FL380. And it displayed step at 1414z and 818 nm or something like that(the figures are not 100% correct since i dont remember the exact values).But it seemed about accurate. But as soon as i reached my crz level the max altitude was still about FL360, maybe 364 or something like that, but the STEP AT changed from about 800 nm to NOW...This has happened every time i have tried to use Step climb, this doesnt seem accurate?! /Niklas Eriksson