Epoxy 3 + 1.5 = ...3.5?

At my work I've been working on a project for our medical drivers for our assembly department. Synthes issued us the glue to use; Masterbond, a two part epoxy. We've found that a 2:1 ratio of the base (3) to the hardener (1.5), and when they're fully mixed and ready to be absorbed into the syringe, I'm never able to get more than 3.5 units in the syringe. Now, to me 3 + 1.5 should be 4.5, right? Can anyone please explain to me where that other 1 unit is going?