So I just did some of my own super-official, totally scientific testing. Dumped out the "empty" ink cart and sure enough, 150ml left in the bag.
In the video I posted above they mention the weight of ink is 1.08g/ml, but they're using water-based ink. In a document I found online from Onyx about ink calculation they mention the weight of solvent ink being 0.7-0.9g/ml, so I went with the average of 0.8g/ml. That means 700ml of ink would weigh roughly 560g. A full ink cart weighs in at 894g, and my empty cart (AFTER dumping the left over ink out) weighs in at 217g. 849g - 271g = 677g. So, by my calculations, there is approximately 145ml of extra ink in the cartridge when new.
Whew! I feel a whole lot better knowing Epson isn't intentionally trying to screw me (at least not with the ink!). Still seems awfully wasteful, and that's a lot of ink ending up in landfills...lord knows I already send a buttload of plastic (in the form of vinyl and substrate scraps) to the landfill.