You are not perhaps confusing the 3.3ml for starter pack cartridges, because I have found that Epson and other makes of cartridge content quantity can vary on which printer model you have or use.
Any cartridge refill company will give you the details for maximum fill, because most of these company's fill to capacity or offer 'normal' or 'higher' capacity fills.
If Epson are reducing their own capacity fill, then perhaps this is to cover and subsidise the cheap cost of the printer?.
As a suggestion, I would check to see if a CISS (Continuous Ink Supply System) or refillables are available for the Epson model you are perhaps considering. I have had Epson's and other printer's run this way, and it can make considerable savings in the long term.