MeanGenes,
The exact mean for 5d6 drop 2 is 13.43.
Why did you use that precise distribution? Other distributions are as logical, and do not produce the same results, for example:
3d6: 1,8333 ; 3,5 ; 5,1666 (sum = 10,5)
4d6: 1,625 ; 2,875 ; 4,125 ; 5,375 (sum 3 best = 12,375)
5d6: 1,5 ; 2,5 ...