I know what you're talking about....but I don't believe you are actually smoking any less than you did with the store boughts. You are just burning them less.
I found the exact same thing when I first started smoking RYOs........I go through less than a pack, usually less than half a pack a day, unless I'm out. And that was doing the exact same things I was doing with the store boughts, but I have been known to go through more than 2 packs a day of those.
There are none of the additives in the RYO tobacco that are found in the store boughts.....particularly the additives that cause them to burn faster. It really has nothing to do with the nicotine contact as far as I have been able to determine.