From the ones done on me for bog standard tests in the last dozen years or more, that wasn't done, but the extra gunk in the vial just might be intended to grab the contaminants as well as stabilize the blood. Or maybe these more standard tests allow for whatever amount of contamination is in the drawn blood, suppose it's 2-3 orders of magnitude less.

They don't throw out the first vial right there in front of you.

When they only take one vial, they'd better not be throwing it out and sending my doctor and myself "results"!

Now, it's possible I'm mis-remembering this, although the protocol includes showing me the vial after the draw so I can confirm it's labeled correctly and I tend to remember additional vials, since that manipulation often hurts more than anything else. I'll pay attention, heck, ask the phlebotomist in a few months during my next annual checkup.