> stated volume is 3.4oz, or 100mL, […] filled it completely, then weighed it. That's 4.9oz.
You've fallen victim to one of the world's classic blunders, the most well-known of which is, "Never get involved in a land war in Asia," but only slightly less well-known is, "Don't confuse the volume measurement 'fluid ounces' with the weight measurement 'ounces', as they are not remotely the same thing!"
To paraphrase, anyway.
If you were to weigh a known volume of water, you could compute the volume of an unknown quantity of water by weighing it, as long as the temperature and pressure of both the known and unknown quantities of water were the same. Otherwise, ... well, you're comparing apples and llamas.