Here is what I think is going on and please let me know if it is not the case and I will investigate.
In the Tools->Options you have the "Volume Calculation" checked. Uncheck it and I believe that you will see the PG/VG percentages act as you described above.
The story behind the "Volume Calculation":
Most people think that PG + VG must always equal 100% even if you have added ingredients that contain something other than PG/VG as their base. Most calculators compute your percentages this way. When the "Volume Calculation" is unchecked, this calculator will calculate your recipe in the manner described. In this scenario the PG/VG values will always adjust themselves to be 100%.
Other people (I am one of them) want to know the actual percentages of PG and VG in their recipe. It is possible that you are adding 45% PG, 45% VG and 10% of other stuff. So... The PG/VG values will only adjust when they go above 100%. Also, the "Precedence" value of PG/VG comes into play. After adding your percentage of ingredient values along with the needed nicotine and PG/VG percentages, if more volume needs to be added to the recipe the PG/VG precedence value will be looked at and it will add the "Precedence" ingredient to bring the recipe up to the needed volume. Does that make sense?
One last note: The "Results" area of the recipe will show you a total of 100% PG+VG if the "Volume calculation" is not checked. If it is checked, the PG + VG may not equal 100%. It will calculate the actual percentages of PG and VG.