A couple of things could be in the works.
One - Did you buy the 40c new or used? Is your 40c a MA (Mass. state) model? If so, maybe it has the 10# trigger pull on it vs. the normal 6.5# trigger pull. Maybe that's why it feels so much different than the stock 6.5# pull of the 40fs.
Two - If you are positive your 40c isn't a MA model and has the stock 6.5# trigger pull, then are you sure than the person didn't put an Apex kit into the 40fs? If so, that drops the pull down to like 2.5# or 3# and that would be a difference between the stock 6.5# pull of your stock 40c.
Again, the only way you'll know these numbers FOR SURE is to use a meter on them to find out the actual weight of the trigger pull for each. Then you will know without a doubt.
When you buy a used firearm from someone, sometimes you never know what you're going to get. That's the only bad thing if you buy it off someone you don't know. So, I would check into those two options above and it might give you some insight.