No shortage of geeks and dorks here to answer all kinds of useless facts and maybe start a good flame war over what temperature we should assume for the specific density of water!
Small errors in the dimensions would make a fairly sizable difference in the capacity. Also your dimensions might be spot on, but the sacs may stretch or bow outward so they hold more water than the strict dimensions would imply. Or to boost sales the manufacturer says they will hold 220# under optimum conditions (e.g, pressurized and about to burst at the seams) that you will never get to in real life. My guess is it's a combination of all three.
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