From my understanding it has to do with the air intake from your pinning up, Mate.

When you're getting ready to pin up, if you pull the plunger back on the barrel to say 2mls and inject this air into the bottle to make it easier to extract the fluid, if the vial is full of liquid the pressure build up COULD pop the cap IF it hasn't been crimped in place properly. And IF you've ever had a cap come off your vial you'll know it's impossible to replace it securely.

So having some space in the vial is not a bad thing.

It's just like a coke can or coke bottle it's never filled to the brim. Why? I'm not sure but thats how it is for some reason

My human grade 10ml bottles hardly have enough room in them for a bb sized bubble when full. I think the diff is my hgrade stuff can be stored longer with smaller amounts of air inside, and the ugl bottles commercially purchased are just larger sized. All my hgrade stuff came in brown bottles too, weird now that I think about it ..