It could be that your charger is cutting it off a tad bit early, but I also think it could be the multimeter that your using is alittle off. Is it a cheap one? I have an Epower which is 3.7V and similar to the kGo but uses non-proprietary
batteries and the same thing happens to me. fresh off the charger my Epower batteries read 4.06-4.07V. I have numerous batteries including a high-drain 14650 battery and they all read the same. I have a cheap digital multimeter that I got off eBay for about $3.50. I didn't buy a used one or anything, just bought it directly from a Chinese seller so it was cheaper. It's the DT-830B multimeter and it's the one that madvapes sells for $10. It works fine and for ohms it's dead on, but for battery voltage I figured out that it's alittle off. I thought it was my charger just doing that at first, but then I used one of my friends much nicer multimeters and it read 4.12V when the same battery at the same time reads 4.06V on mine. 4.12 is perfect bc charging it to a full 4.2V can actually slightly overcharge some batteries and potentially damage them so that's why I'd rather have them come off the charger closer to 4.1V.
So try checking them with a different multimeter and see if you get a different reading. I know now that if the battery says 4.06V on my multimeter than it is charged to exactly where it should be