As KevinC mentioned, you can easily increase your precision by increasing the volume of water to be tested.
The gH and kH test kits work on the basis of titration. As KevinC also mentioned, if your 4 dkH reference solution is slightly off (i.e. 4.1 dkH), then it will require that 5th drop to change the colour to a bright yellow.
As for where your error is coming from, I would suspect the sodium bicarbonate as well. Despite it being a brand new box, it is not completely air tight, and there are surely impurities left over from the manufacturing process, etc. However, since it is going to go into a drop checker, which is not the most precise tool anyway, you can probably get away with the reference solution you already have made up.