compile the same project several times get several different timing results, why?

Yes, it regenerate sopc system each compilation, and there is system id component in the sopc system.

I run a make file which generate project file, generate sopc system and compile. Each time I run the make file (before next compilation, all generated files are deleted), it gives different timing result.

In order to check it is the matter of regeneration of sopc system or not.

I open the generated project file in Quatus (all generated files are kept), I recompiled by double click compilation design in Quatus GUI. The timing result is still different with previous result which is compiled by make script. But if I recompile in Quatus GUI again, it will stay the same. I do not understand, are there any difference between Quatus GUI and script command?

By the way, I installed quatus 11.1, I do not install service pack 1, but the builder is 173.