Synthesis using Synopsis
Synthesis tools use various techniques to optimize the results, particularly with regard to placement. Some of those techniques, such as simulated annealing, make use of random numbers. Apparently, the random number generator is seeded with a fixed value when you launch the tool, but additional randomness is incorporated on successive runs.

The bottom line is, all of the different results are functionally correct and meet your timing constraints, but simply have the delays distributed slightly differently. There's really no reason to prefer the "first" run over any of the others.