I wonder at what concentration (proportion) the effect is greatest; presumably not too far from the 4% nic that gives the 10% increase in density. Might the effect become even stronger at say 10%? Possible, and would begin to explain some of the discrepancies found in some high strength nic base liquids; depending on how the dilution was done.
If the shrinkage was fast (immediate; as one would reasonably expect - why would it take time?*), this would lead either to over or under strength liquid depending on what liquid was being added (assuming the added liquid was simply added to a predetermined overall volume). Otherwise, the shrinkage would effectively lead to increased strength by volume; (anyone checked if their 30ml / 100ml of liquid is actually of lower quantity?; of course a vendor might reasonably be expected to err a bit over with this as it is not critical).
After all, when we say that an polar inorganic liquid is hydrophilic this is not just about solubility in water but an attraction for polar molecules whch 'fit' in the htdrogen bonding sscheme; water molecules, being very small and of simple shape are an ideal; but nicotine molecules seem to be a good 'fit' too.
It is certainly a different mechanism to dissolving salt in water whereby the volume of the solution is barely increased as the ions just 'fill in the gaps'.
The figure for the 36/40+ liquid is ~8% more dense (similar to the change in density of ice to water; i.e. very significant).
Certainly this is cause for further analysis ...
* If this is a slow process, the reason might be, strangely enough, the absorption of water - increased mass with little increase in volume. Though this would not alter the nic strength by volume. So it is most likely instant volume shrinkage due to enhanced H bonding.
Interestingly, life depends on H bonding making water ~15% denser than it would be otherwise; without this, water would not remain liquid to anywhere near 100C and life out of water at least probably could not have evolved..