It occured to me after writing the last response that some people may actually benefit from a visual metaphor to help understand the way that a mix works, especially given the fact that most of us on this board didn't grow up working with metric measurements.
Imagine that each ml of liquid is a bucket filled with 1000 marbles. This is the same as the 1 gram (1000 mg) of total mass to each milliliter.
100mg would be 100 nicotine marbles in 900 VG/PG marbles (lets ignore flavor for now

)
Each bucket could be broken down into 20 scoops (drops)
If you took a scoop of these marbles, assuming they were very well mixed, you'd get 5 nicotine marbles, and 45 VG/PG marbles.
If you drop that scoop into a fresh bucket, and fill it up the rest of the way with 19 scoops of VG/PG marbles (950 more of them), you still end up with only 5 nicotine marbles, and now 995
total VG/PG marbles. If you evenly mixed it, each scoop of this new bucket will have less than 1 nicotine marble in each scoop (since mg isn't really a single object, you'd have to imagine partial marbles... that's where this metaphor would break down a bit).
If you had a premixed liquid, at the 12mg/ml mentioned earlier, that'd be 12 nicotine for every bucket. 3ml would be 3 buckets, so 36 total nicotine units.
I hope that you can also see my earlier point that getting even "scoops" would be very difficult, just the same as getting even drops.
Does that help anyone?