Not too sure, but conductance may also be due to the concentration of the different dissolved species in the salt solution. All salts have different solubility, with some being more soluble than others. Hence, for example, the concentration of dissolved chloride ions would differ between a sodium chloride and magnesium chloride solution. It would not be a simple "double" the concentration in sodium chloride solution for the magnesium chloride solution, if you know what I mean.
On a sidenote, the conductance may be also due to the SUM of the different dissolved species in the solution. For example, if you were to dissolve 1 mole each of sodium chloride (58.44 g) and magnesium chloride (95.21 g) in 1 litre of water, the total concentration of dissolved species would be 2 moles per litre for sodium chloride, and 3 moles per litre for magnesium chloride. But this is assuming both salts have the same solubility.
Umm, hope this helped. Reply if it doesn't and I'll try to simplify further.