The 15:16 is due to the difference between the clock speeds of the memory and the FSB. I see that you have an nForce 750i SLI motherboard, and if I remember correctly, then I think that achieving a 1:1 ratio was not nearly as important as achieving the tightest timings. I had an nForce 680i SLI board and I always had a ratio like 15:16, but I never had any problems due to not having a 1:1 ratio.
I think that this was just exclusive to the nForce chipsets because I kind of remember people saying that it was more important to shoot for a 1:1 ratio if you had an Intel chipset as opposed to an nForce chipset. So, I never made any attempt to get a 1:1 ratio (plus, a 1:1 ratio would have given me an overclock for both my CPU and my memory that I didn't want).