For all those people who try to get every last drop of reagent A out of the bottle when B & C are empty, keep in mind that strategy is going to screw up your calibration process (since you only get one bottle of calibration fluid in each package of A, B, C refill kit), and you will also end up with a surplus of unopened reagent A bottles over time since the individual reagents are not sold separately. It's not going to save you any money in the long run.
The bottom line is that the consumption of reagent A varies based on your water's alkalinity level. Those running very high alk will burn through more reagent than those with low alk levels. To account for this, Neptune supplies more reagent A than necessary for most tanks, leading to some reagent A being left in the bottle when B & C run out.
When B & C are empty, then it's time to swap out all 3 reagents, even if A isn't completely empty.