First of all, I'm not familiar with your method of expressing the nic content of your e-liquid. I'm familiar with the 3mg, 6mg, 12mg, 18mg, and 24mg method.
Are you comparing the nic levels with the same juice? (same flavor and brand?)
Although the nic concentration has the most effect on throat hit, the amount of Pg (propylene glycol) in the Pg/Vg ratio also carries some influence. Are you using the same juice attachment, at the same power level (watts or volts)?