There is a huge factor difference in how much nicotine we all absorb. I only need 6mg strength liquids. If I use 18mg I can only have 0.1ml of liquid a day or I will feel awful. I use 1 - 2ml of 6mg liquid instead.
On the other hand, I have seen some who use 32mg and up and more than 2ml a day of it. At the highest I've seen someone who uses 52mg so a normal cartridge would not be enough for them.
The way they worked out how many cigs are in the cartridges is totally wrong as it doesn't factor in absorption rate at all so it is impossible to say what it will equal for you.
You're seemingly confusing absorption rate with tolerance. If you had a very low absorption rate, you wouldn't feel awful using 1ml of 18mg a day, even though your tolerance level hasn't changed. If you had a higher absorption rate and the same tolerance level, you might feel awful with 1ml of 6mg.
The people who use 32 and 52mg might actually have exactly the same tolerance level, but far different absorption rates.
In the end, it doesn't matter when determining what a cartridge is equivalent to. When you're calculating that figure, you need to assume a certain "average" absorption rate. And you need to assume a particular nic content in the cigarettes you're using in your definition of a pack of cigarettes. Whether or not that amount is likely to satisfy any particular vaper or not is irrelevant.
What these companies probably do is count the entire nic content and compare it to some hypothetical pack of cigarettes. The absorption rate of vapers is somewhere around 30-50%, so right off the bat, they're overestimating by a factor of 100%. If your rate is 30%, they're off by a factor of over 300% and a cartridge may be equal to only 7 of their hypothetical cigarettes. If their hypothetical cigarettes are ultralights, then the number plunges even further and, if you smoke Marlboros, it may feel like 3 cigarettes to you.
That wouldn't mean they're lying by a factor of 7. They're free to choose any comparison cigarette they want for their hypothetical, but they need to spell it out if they expect someone to be able to estimate how many cartridges they'll need.