Well, because the density of liquid ammonia (.68 g/mL) and liquid water (1.00 g/mL) are different. So if the solution is close to pure ammonia, you divide the first number I gave by the %age ammonia in solution (If it's greater than, say 50%).
Otherwise, you have to take the density as closer to water. That's why the numbers are slightly different... about 4 mL vs. 5.5 mL or so.
Anyway, both numbers are pretty close to a teaspoon. So at least you have a rough idea about how much to add.
