Why is chip-time not considered as the official race-time from which finishing positions are calculated? (summary answer)
Based on an extract from IAAF Rule 165.24 in Regard to Transponder Timing Systems (page 114 and 115 of IAAF rule book):
The official race time shall be the time elapsed between the firing of the starting gun and an athlete reaching the finish line (=gun/gross time). However, the time elapsed between an athlete crossing the start line and the finish line (=chip/nett time) can be made known to him or her, but will not be considered as an official time.