zedborad ddr3 dq delay

Hi all,
 When I read the Zedborad_HW_UG_V2.0.pdf,I found the Propagaton delay of the DQ[7..0] is 160ps/inch(Table 2 -DDR3 Worksheet Calculations).
 Also I found the signals of DQ[7..0] are not in the same layer (Gibber file of the zedboard),for example the dq2 is in the top layer and the dq4 is in the Third layer mainly.
  The propagaton delay of the trace in the top layer and in third layer is not the same. I calculare the delay time according to the pcb information of zedboard.  The propagaton delay in the top layer is 149ps/inch and  in third layer is 173ps/inch. Also the dq4 has two holes and each adds about 17ps delay。if we assume the net length of the dq2 and dq4 are equal,then the delay time of dq4 and dq2 is (173-149)*netlength + 17ps*2. The netlength is about 51mm,so the dalay time between the dq2 and dq4 is about 80ps.
  Could someone tell me why the zedboard is still working well and  what is the max skew of the dq group of the zedboard.
 
   thank you