Well, the code of pre-synthesis is the final code, once the design meets specs. But if you go to PnR and the find a problem in the design, you have to come back from the PnR step, and fix your pre-synthesis code(RTL), synthesize it again, PnR it again. In some cases if the porblem is not a major one, the post-synthesis code i.e 'netlist' may be changed and only PnR is done again. In some very rare cases, if the problem is even more simpler to fix, it might then be fixed at the layout mask level, so that you dont have to do even th PnR again. But it all depends upon the change/fix that is required.
In summary, the pre-synthesis is the final code but it may or may not correpond exactly to the layout if some post synthesis fixes i.e ECOs have been done.
Kr
Aviral Mittal.