Sounds like you didn't do a "Between the Lines" measurement.
To Dispense 1ml, try drawing 1.5ml into the Syringe and then Purge any Air in the Barrel until you have 1.2ml in the Barrel. (The 1.2ml Doesn't matter and it can be Anything Greater than 1ml). Now Dispense the Flavoring until you get to .2ml on the Barrel and Stop.
1.2ml - .2ml = 1ml
A Syringe may be Calibrated for a Given Needle Length and Gauge. But if a Person uses a Bigger Gauge and or Length, they are going to get More than 1ml if the go from 1ml on the Barrel to Full Depression of the Plunger. And Vica Versa if the Needle is a Smaller Gauge or Length.
AFAIK, this is the best way to measure with a syringe. However, since I use the same syringe for all flavors in a mix, I don't like expelling unused flavoring back into the original bottle as it may be "contaminated" with traces of the previous flavor.
I measure out the amount of PG I need in a recipe, and then I set a bit of it to the side. I use this PG to prime the needle and syringe; I draw up some PG, flip the syringe upside down, expel all of the air and just a drop or two of the PG, then flip it back over and flush the rest of the pg back into the amount I've set aside.
Since the needle is now full of PG, drawing flavors up from 0 to whatever measurement I need means I get that amount of flavoring, plus a needle full of PG. I expel that flavor into the mix, wipe the outside of the needle with a PG soaked paper towel, and then move on to the next flavor. Since the needle is still full of fluid, there's no need to start the process over again unless I accidentally draw some air into the syringe. (ETA: If I'm using some really strong or contrasting flavors, I'll flush the syringe with a round of PG, or even change syringes. That's pretty rare though; I usually only do this with potent tobacco flavors, or when I'm using espresso, etc.)
At the end of the recipe, I draw the remaining PG I've set aside, and then put it into the mix. Finally, I draw a syringe full of air, allow the little remaining liquid to pool at the needle end of the syringe, and then blow it out into the mix.
This is not as accurate as the method listed by zoiDman, but it is pretty consistent regardless of the needle gauge or bore.
For the record, I really like the concept of mixing by weight. However, having reloaded a lot of handgun ammo, I'm not too impressed by most of the inexpensive scales out there. They're OK for measuring a given weight (placing a weight on the scale and taking it off), but they have not,
in my experience, done well with gradual and minor adjustments in weight.