This is a bug in TRKSD00 class. The last step length is added twice in UpdateHit() and in DumpHit() to a Mokka-hit-length. If there is only one Geant-step in a layer then Mokka-step-lenght = 2*Geant-step-length. I have written info about it in Mokka "issues tracker": http://polzope.in2p3.fr:8081/MOKKA/issues-tracker/4