Variation in sof file size

Hi,

The size of SRAM Object File (sof) generated by quartus seems to be changing even for repeated invocation of the same project with same settings. The difference is in few bits. I do not have the board yet so I could not test its correctness.

Based on my experience with other FPGA devices, sof file remains same for a particular device and is independent of logic utilization. At least in this case it should remain constant. Is quartus doing something different? I am using Quartus 12.1

Thanks