Yesterday I measured the density of a 36 mg PG liquid known to be only nic and PG to be 1.128 g/mL. The density of pure nicotine liquid is 1.009 g/mL, and the density of pure PG is 1.036 g/mL. Thus if there is no change in volumes upon forming a solution, the final liquid density should be somewhere between these two densities of the pure substances.
What this implies is if a 250 mL 36 mg liquid is made from .036*250/1.009 = 8.92 mL pure nicotine, and the volume is brought to 250 mL with the remaining 241.08 mL, when the solution is finished forming, its volume will only be 223 mL, and the actual nic concentration will be about 40 mg/mL, not 36 mg/mL.
I do not work with pure nicotine, and have not made any liquids from PG or VG and pure nic myself, so I cannot verify that the solution actually did contract. I only measured the density of a known liquid sent to me by a vendor. That liquid titrated to 43.5 mg, so perhaps this is was much of the issue, along with the usual error associated with any volume dispenser. Not saying who the vendor is, since there have been no complaints, he just wanted me to test his. He makes liquids individually per bottle, using a volumetric syringe to deliver the pure nic to the bottle, dilute it up to the level for that bottle with PG, and shakes it up for a while.
Is such a contraction known with these liquids? Something is increasing the density, and a post diluting contraction is the only thing I can come up with. Such an effect would not surprise me, since both nic and PG are hydrogen bonding and a solution may disrupt some of them. For a similar reason a volume of solid ice will contract when it melts, the result of some of the H-bonds breaking in the melt.