If a maxed stack inserter by a yellow belt has a capacity of 6.79/s, wouldn't that mean that the third inserter would be active ~10% of the time since a yellow belt is 15/s? I've ran a full yellow belt for quite a while and the third chest doesn't even have 1 stack yet. This tells me that the third inserter is active 1% of the time based on the products finished statistics. Could you double check the calculations for a stack inserter over a yellow belt?

Numbers for taking from belt are approximate. And even if I were somehow to implement the exact simulation for it, the number would depend on how filled the belt is. Use the stopwatch tool if you want to get actual, in-world numbers.