These cards are slightly different in production ages the earlier produced one also is running on PCIe X 16 3.0 @16 using the earlier version 1.1 of this standard, the more newly produced one is PCIe X 16 3.0 @16 using the later version 3.0 of this standard, this is not of any real-world significance. The older one uses Hynix GDDR5 while the newer one uses Samsung GDDR5 memory.  I suspect the differences in bios are only due to the differences in the production age as noted above.  The differences in your cards performance should be essentially nonexistent except for the luck of the draw in silicone, and unless you are planning some very very serious over-clocking in SLI use you will never need to worry.  If a bios update is ever issued you could flash both cards and then they would have the same bios numbers, but you really shouldn't unless you are having specific issues addressed by the bios revision.