The accuracy of salt tests are not very good. The one on the IC40 is apparently +/- 500 ppm, and the one you're doing by hand is +/- 200 ppm. So there's a really big range. Your salt could actually be 3200. In my experience with my SWG my K-1766 is usually higher than the generator says. So I assume somewhere in the middle is probably about accurate. Right now mine is reading something like 3800 by hand and 3200 by the generator.
I'm not sure what that means for you though. Should you add salt? Or is the unit bad? I don't really know. If it were me I'd add salt assuming I was around 3100 probably, enough to get to 3400 or whatever is ideal for the IC40, and see if it seems happy and generates the right amount of chlorine that I'm used to. But I'm not an expert.