The 5x M9 salts recipe here seems to be almost identical to the 10x M9 salts protocol we use in our lab and which I also found elsewhere (see e.g. http://www.changbioscience.com/protocols/recipe/M9salts10X.htm and http://www.atcc.org/mediapdfs/2057.pdf). It could be that bacteria don't mind if they get the double of salts, but it might nevertheless be interesting to know what conc. of salts would be best to use... (MDolinar)
I think that this may be explained by the additional mass of 7 waters associated with each Na2HPO4. The molecular weight of Na2HPO4 is about 142g/mol, while that of is about 268g/mol. So 5x M9 salts could be made with either 64g/L (according to Sambrook) or 33.9g/L Na2HPO4--the concentration will be the same. Then if you double this for 10x salts, you would end up needing 67.8g/L, which is close to the 60g/L given in the standard 10x recipes. I can only wave my hands at the 7.8g discrepancy and say that someone goofed or altered the recipe slightly.