op, I received the same question yesterday.
See this for my reply:
Calculator not adding up??
Answer:
The calculator came up with the correct amount but it sounds like you used a dropper to measure your liquids.
The dropper method is not recommended as I stated in a few posts.
See second from the last paragraph at:
DIY eJuice Mixing
The reason the dropper is not accurate is because each liquid will have a different consistency. VG drops are larger than a PG drop and different droppers give different drop size.
The drop box that shows "20" can be changed but remember if you use VG, PG and different droppers it still won't be accurate.
I always use a syringe. My preference is a 3ml syringe with a 12ga blunt tip for thick liquids such as VG and Nic that has a lot of VG and an 18/20ga blunt tip for PG based liquids.